Special relativity and time dilation

Now that we know what a theory is, we can tackle one of the biggest theories in physics: general relativity, Einstein's masterpiece. We should, however, start with special relativity to prepare the playing field a bit. This post (and the next one) will be a review of the things we discussed on Periscope recently. There will be some maths, just in case you want to check the numbers I give, but you can happily skip all the maths and still understand what I'm saying.

In order to understand Einstein's theories, we need to understand the context in which he developed them; which means we need to start with Newton.

1. Newtonian gravity

Newton invented calculus, a law of
gravity, and ridiculous hair styles.

Newton, as well as inventing calculus, came up with an idea in 1687 that was revolutionary for his time: things fall down. But they don't just fall down, they fall towards each other. Specifically, he realised that any two objects in the universe - no matter how big or small or how far away they were - would attract each other via a gravitational force, as long as they had mass.

This gravitational force is attractive, universal, proportional to the mass of the objects, and inversely proportional to their distance. Which means the closer together and the bigger the objects are, the more gravitational pull they feel.

That is why we are stuck on the surface of our planet: Earth is huge and very close to us, so it attracts us. The Sun is huge and relatively close to Earth, so it keeps Earth in a stable orbit. Gravity is also very weak, which is why something as small as a magnet can pick up a paper clip that is being held down by the whole planet's mass.
Newton's law of gravity was, and still is, very useful for describing gravity in everyday life, but it had it's problems. The first problem Newton encountered was that he couldn't explain why gravity exists, nor how it could affect things at a distance. It also had serious problems explaining the movement of things that travelled really fast, such as light beams. It was incomplete, but it was the best he could come up with in his time.

2. Special relativity
In the second half of the 19th century, scientists were trying to find an aether; a mysterious medium through which light (and gravity) propagated. In this point of view, the laws of physics would depend on how people moved relative to this aether, despite the fact that this contradicted experiments. Then along came Einstein to shake things up a bit, he decided to accept the experimental results as true, and modify the theory, and in doing so he removed the need for an aether.

The first concept we need to understand is that there is no such thing as absolute motion; everything moves relative to one another. As an example, imagine you are on a train travelling at 100 km/h. You could quite happily argue that you are at rest, and everything else is moving. If you were to throw a ball out of the window at 10 km/h in the direction the train is moving, you would see the ball as moving at 10 km/h. An observer outside the train, however, would see the ball moving at 110 km/h, as they would have to add on the speed of the train. Motion depends on the observer; it depends on the frame of reference from which you measure.

Next we can introduce the concept of inertial frames of reference: these are just systems that move at constant velocity, with no acceleration. If you are in a car travelling at 50 km/h constantly, you are in an inertial frame. However, when you slow down at a traffic light, you are changing your speed, so you are no longer in an inertial frame.

Armed with these pieces of knowledge, we can now formulate the postulates of special relativity, as Einstein did in 1905. Note that a postulate is something we use to build a theory; they are facts that have been proven to be true, but we don't necessarily need to know why they are true.

1. The laws of physics must be the same in all inertial frames of reference (thus eliminating the idea of an aether).
2. The speed of light in a vacuum is the same for all observers, regardless of the velocity of the source/observer.

These postulates matched all the observed phenomena, and changed the way people thought about motion. From these two postulates, we reach an obvious conclusion: the idea of simultaneity is relative. If you measure two events as happening at the same time, it doesn't necessarily mean other observers will also see these events happening at the same time. Einstein had a useful thought experiment to illustrate this. Imagine you are on a moving train (at a constant speed!), standing directly at the centre, and you fire two laser beams in opposite directions.

You will see the laser beams reach the ends of the carriage at exactly the same time, as seen in the image on the top. An observer outside the train, however, will see the train as being in motion, so for them the light beams will not reach the ends of the carriage at the same time. This is because from their point of view, the light beam moving in the direction of travel has a greater distance to cover, because it has to 'catch up' with the movement of the train, as seen in the bottom image.

The next big question we need to ask ourselves is how can the speed of light be constant? Let's imagine we are on separate rockets, and for now we'll work from my frame of reference. My rocket is at rest, and your rocket is moving past me at 0.5c (half the speed of light). In your rocket, you are bouncing a light beam off two mirrors; one on the ceiling on one on the floor of your rocket. You will measure the light beam as travelling at the speed of light. From my frame of reference, I will also measure the light beam as travelling at the speed of light, even though the light is travelling a greater distance (because the mirrors are 'running away' from the light beam at the speed the rocket is moving.

Image source: How to: Special Relativity

In the above image we can clearly see that from my point of view, the light beam has a greater distance to travel. Velocity is always defined as distance

travelled over time it takes, so if you and I measure the same velocity, but a different distance, something else also has to change. Specifically, the time it takes to make the journey also has to change.

3. Time dilation in special relativity
If we impose that the speed of light has to be constant, and different observers see light travelling different distances, the clocks must also tick differently. This is the main idea of time dilation: time does not pass the same for people moving at different speeds. Recovering our example of the rockets above, if you are travelling at 0.5c, I will see clocks on your ship as running slower. By the time I've seen your clocks tick one second, mine will have ticked 1.15 seconds. You will still feel time as running normally, but I will see time passing slower for you.

We can see some specific examples of this if we look at the formula on the right: if I see you travelling at 0.5c, one second for you is 1.15 seconds for me. If you are travelling at 0.9c (90% the speed of light), one second for you will be 2.3 seconds for me. If you travel at 99.9% the speed of light, one second for you is 22.3 seconds for me. The faster you travel, the more I see your clocks slowing down.

This effect, however, is not enough to keep the speed of light constant; there is an analogous effect known as 'length contraction', where lengths increase for observers moving fast (1 meter for me is 1.15 meters for you), but I won't go into that effect today. This idea of time dilation for moving frames of reference has some important consequences:

• Space and time are not separate entities, they are united. Einstein replaced the notion of space and time, and instead introduced spacetime; two sides of the same coin.
• Time slows down as we approach the speed of light, so if we reached the speed of light, time would stop completely. You can extrapolate to say that if you move faster than the speed of light; time would move backwards. But nothing can travel faster than the speed of light; as such clocks can't tick backwards. This means that time travel to the past is not possible.

Don't believe me? Check the maths!

Another argument one could naively make is the following. If I'm travelling at 0.6c, and you are travelling at 0.6c in the opposite direction, surely I can argue that you are travelling away from me at 1.2c, therefore you are travelling faster than the speed of light from my frame of reference. This was true from a classical approach, but completely wrong from a relativity approach: you can't just add velocities together like that. When dealing with high speeds, we need to take into consideration time dilation and length contraction. If we take into consideration these effects, you would actually see me moving away at 0.95c, so still within the allowed speed limit.

You might be thinking at this point that we've done everything from my frame of reference. What happens if we change to your frame of reference? Actually, as we've seen that motion is relative, you could argue that you are at rest, and my rocket is moving. In this case, you will see my clocks as running slow. Time dilation caused by motion is also relative: you will see time passing slower for me, and I will see time passing slower for you, which leads us to an interesting paradox.

4. Twin paradox
Imagine we have twin sisters, Elizabeth and Laura. Laura leaves on a round-trip mission to Alpha Centauri, the closest star to the Solar System (except the Sun, of course). This star is 4 light years away (this means it takes light four years to make the journey), and Laura will be travelling at a constant speed of 0.8c. Elizabeth will be staying on Earth, monitoring her sister's journey.
In Earth time, which is the same as Elizabeth's time, the round trip takes 10 years:

$t=\frac{2*d}{v}= \frac{2*4}{0.8}=10$y.

We are interested in how much Laura will have aged from Elizabeth's point of view. We can use the first formula I introduced above to calculate this (or skip the calculation and trust my numbers!):

$t = \frac{t'}{\gamma}= t' \sqrt{1-\frac{v^2}{c^2}} = 10*\sqrt{1-\frac{(0.8c)^2}{c^2}} = 10*0.6 = 6$y

This means that from Elizabeth's point of view, Laura's ship has only felt 6 years, even though Elizabeth felt 10 years. From Elizabeth's view point, when they reunite on Earth, Elizabeth will be 10 years older, while Laura will only be 6 years older.

Now comes the paradox. If we said there are no preferred frames of reference, Laura could argue that she is stationary, and Elizabeth is the one moving. Therefore, from Laura's point of view, she will age 10 years, and Elizabeth will only age 6 years. They can't both be right, so who is correct?

While this result might seem paradoxical, it actually isn't. Laura is naively applying the conditions of special relativity, however we stated above that special relativity works in inertial frames of reference. Laura's ship, however, is not an inertial frame of reference. This is because her ship turns around, and during that time she is changing her velocity. Changing velocity means acceleration, which means it's not an inertial frame.

So how do we study acceleration? Einstein saw that his idea was valid for inertial frames, so he tried to apply it to non-inertial frames. To do that, he needed to think bigger: he needed general relativity. For now, rest assured that the paradox does have a solution: Laura will be younger than Elizabeth after the trip, in the same way that astronauts on the ISS age slower than people on Earth. We'll find out how to deal with acceleration in the next post.


In the second part, we'll discuss general relativity, time dilation from a GR point of view, and we'll revisit the twin paradox, armed with new tools to tackle it better. I'll also discuss some specific examples like the ISS, a possible Mars colony, and of course, everyone's favourite time dilation movie; Interstellar. Be sure to check it out!

What's in a theory?

My objective with this blog, apart from having a place where I can ramble about cosmology, is to make science understandable. But if we want to understand each other, we first need to make sure we are speaking the same language, and that often doesn't seem to be the case. I'm not referring to all the scientific jargon that we scientists like to throw around, I'm talking about those words that people use everyday but which have a very different meaning when used in a scientific context. The best example of this is the word theory.

Every time someone misuses the word 'theory',
Schrödinger puts another cat in a box. Maybe.

I'm sure you've seen the situation before: a scientist explains a scientific theory, and someone replies with 'but it's only a theory'. At this point the scientist will cringe, shake their head, and decide it's not worth pursuing the issue. But it is worth pursuing.

The word theory is not a bad word, in fact it's one of the best words we can have in science. Here's the definition:

"Theory (n): A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena." -  Collins English Dictionary – Complete and Unabridged, 12th Edition 2014

Now that we know the definition, let's do some science!
We start with a fact. This is just an observation about the world, that we are very sure is true: it's something we can all observe and agree on. Well, probably all of us; there's always someone who likes to deny the facts ('What do you mean the sky is blue? It looks decidedly green to me'). Let's use an example: I put a coffee cup on the desk, I leave the room for a few minutes, and when I come back the coffee cup is on the floor broken (don't worry; it was empty, no coffee was wasted in this experiment!). Fact: my cup is broken on the floor. So we ask a question: why is my cup on the floor in pieces?

Next we move on to a hypothesis. A hypothesis provides a possible answer to our question regarding the observed fact. We can come up with several different hypotheses to try to answer this question. For our broken cup situation, we could have a few possible answers:

1. Someone pushed the cup off the table
2. There was an earthquake/natural phenomena and the cup fell
3. The table is wonky and things tend to fall off
4. The cat pushed the cup off the table
5. The cup thought it was a bird and decided to try to fly.

We use other connected facts to eliminate some of the bad hypotheses. In our example we can use the fact that cups don't think they are birds to discard number five. We can also use the knowledge that there is no one else in the house to discard number one. Number two seems unlikely, as I would have felt the earthquake from the other room. So now we have to do an experiment to test the remaining hypotheses.

For our example, I set another cup in the same place, and I move away to observe. Will the cup fall off because the table is wonky? After several minutes, the cup hasn't fallen. So it seems we can discard the wonky-table-hypothesis. It doesn't take long, however, for a cute cat to jump on the table and knock off the cup, which falls on the strategically placed pillow I set out for our experiment. Maybe we should have left the cat in the box. All the evidence seems to support the fourth hypothesis: the cat pushed the cup off the table. Hypothesis confirmed.

Another very important concept in science are laws. Laws can generally be written as mathematical expressions, and they describe what happens, based on a series of repeated experiments. Newton's law of gravity can tell us how quickly the cup fell off the table. The conservation of mass law tells us that if we add up the masses of all the pieces, we should find the mass of the original cup.

And now we put it all together. We use multiple confirmed hypotheses, based on countless experiments, repeated time and time again, with all the laws describing the experiments, and we group it all together to form a theory. A theory contains laws, confirmed hypotheses and facts, and most importantly, a theory can make predictions about what other facts we can observe. It looks something like this:

And this is how we do science. If a theory makes predictions which don't fit the facts, the theory will be modified. If we find a fact that contradicts the theory, the theory will be modified. Science is constantly changing, and theories are being tested constantly. A theory is the best thing we can have in science, it's telling us 'I am the best model to explain the observed facts, I can make predictions, nothing we have found yet contradicts me, and I have passed every single test you have thrown at me'.

It's not 'just a theory'. It's human beings looking at the universe and saying 'I see what you did there, and I am able to understand it'. And that's awesome.

Still not convinced? Maybe Joe Hanson from It's Okay To Be Smart can help you out:

Cosmology FAQ Part 4 (The End)

As you've probably already heard by now, I recently did a 'Cosmology 101' on Periscope, a two-hour long scope in which I addressed the 25 most common questions in cosmology. Don't worry if you missed it, you can just read these posts.

This is the final part in this series. You can read the previous sections on the Big Bang, the history of the universe, and the present state of the universe. Today we'll answer the big questions.

As always, don't worry if you don't understand everything in this post. This is meant as an overview to give you a general idea of what we do in cosmology. I encourage you to let me know if you see anything mentioned here that you would like to know more about, or that you think is unclear.

19. What is a black hole?
If you throw a tennis ball upwards, you know it's going to fall back down. This is because the gravitational force pulls things together. But if you were able to throw the ball fast enough, it would have enough energy to escape the gravitational pull of the Earth and launch into space. The speed needed to beat the gravitational force of an object is known as the escape velocity. A black hole is an object whose escape velocity is larger than the speed of light. This means that not even light is not travelling fast enough to escape the gravitational force of a black hole. And as we know, nothing can travel faster than light in a vacuum, which means nothing can escape a black hole.

But don't worry; we (probably) aren't going to be swallowed by a black hole. This is because the gravitational force decreases quadratically with distance: if you get ten times further away from an object, the gravitational force is a hundred times weaker. As such, a black hole has an event horizon: an imaginary line that marks the point of no return. If you are further away than the event horizon, you can still escape the black hole's gravitational pull. But you really don't want to cross that horizon: once you do, there's no coming back.

Simulation of a black hole, by Alain Riazuelo

Physically speaking, a black hole is generally formed by the death of a star: when a big enough star reaches the end of its life it can explode as a supernova, leaving behind a really massive - but quite small - object. If two black holes collide, they can form a bigger black hole. 'Normal-sized' black holes are generally between 5 and 35 times the mass of the Sun. And then we have their older brothers: the supermassive black holes, which can be million times more massive than the Sun, and play a really important (but not fully understood) role in the formation of galaxies.

Mathematically speaking, black holes are regions where the curvature of spacetime becomes infinite (Einstein taught us that this curvature is what causes the gravitational force). These regions are known as gravitational singularities, which leads to an important question...

20. Was the Big Bang a Black Hole?
You've heard me before describe the Big Bang as a singularity, and I've just described a black hole as a singularity, so it is logical to ask if they are the same thing. But actually, the Big Bang was nothing like a black hole. The main difference here is we believe the Big Bang was a singularity, with time and space existing inside the singularity. So the Big Bang was a singularity of space and time, whereas a black hole is a singularity in space time.

21. Is our universe unique? What is the multiverse?
There are many different theories in physics that support the existence of multiple universe. Some people claim there are an infinite number of universes, each with different laws of physics, and we just so happen to live in the region which has the conditions necessary for us. Other people claim that when the universe inflated, it did so in little 'pockets' with each pocket universe evolving with a different amount of inflation, leading to very different universes. In any case, we have no way of accessing these postulated alternative universes, or obtaining any information from them. Therefore, it is not a productive question: it deals with something we don't know nor have any ability to know, and that is not likely to change soon. It is an interesting thought exercise, but it's not really a useful question from a scientific point of view.

22. What is dark matter?

This is one of the biggest open questions in physics. We are fairly sure that 25% of the universe is made up of dark matter, but we don't yet fully understand the nature of this mysterious matter. It's a type of matter that doesn't interact with the electromagnetic force; this means it doesn't emit, absorb, or reflect light, so we call it 'dark'. We are also quite sure it doesn't interact via the strong force, which means it doesn't form big clumps: the dark matter particles don't bind together. It does, however, interact via the gravitational force. And this is how we know of its existence.
There are several main evidences for the existence of matter we can't see. One such example is the rotation curves of galaxies. In a normal galaxy, we would expect the stars further away from the central bulge to move slower; in the same way that Neptune moves around the Sun slower than the Earth does. However, when we take enough measurements of galaxies, we actually see that the outer stars are moving at the same speed as some of the closer stars. The following plot shows the velocity (vertical axis) and distance (horizontal axis) of the stars in a galaxy.

The only way we can explain this with our current laws of physics is by assuming there is more mass located in the halo of the galaxy: a huge amount of matter that we can't see.
Other evidences for dark matter include merging of galaxies, and weak lensing, which I'll elaborate on in a future post, but the idea is the same: if our theories of gravity are correct, the visible matter in the universe is not enough to explain the observed phenomena. There are hundreds of experiments currently trying to find what we have not yet been able to see.

23. But what if we are wrong about gravity and DM is not real (MOND)?
There are some physicists who believe that instead of assuming dark matter is real, we should assume our laws of gravity are wrong. There are some who argue that instead of looking for 'invisible' matter, we should instead modify our theories of gravity (MOND). While these models are able to make some predictions, they can't explain observations we have made in galaxy clusters, nor can these models be used (yet) to build a complete cosmological model. They have often been described as being 'ad-hoc', with more elements added to an ever-complicated model to try to fit the evidence. These models have lost favour in recent years, but some people still pursue them.

24. How will the universe end?
There was recently a great review about this topic on the BBC. There are several different possibilities as to how the universe might end, although the data seem to favour the first two.

• Big freeze/heat death. The expansion of the universe is currently accelerating. If this carries on, the universe could expand forever. This means everything would gradually grow further and further apart, while cooling down and approaching absolute zero. Entropy would reach its maximum, which means there would be no exchange of information nor exchange of heat. Everything would just freeze. In this scenario the universe ends up very cold, dead, and empty.
• Big rip. If the acceleration of the universe increases, perhaps caused by an even stronger type of dark energy, the rate of acceleration could increase so much that it could overcome the pull of gravity on ever smaller scales. As a result, all material objects in the universe, starting with galaxies and eventually all forms of mass, no matter how small, will be ripped apart; reverting back to unbounded elementary particles and radiation, shooting away from each other.
• Big Crunch. This is a very nice, philosophical view of the universe. In this scenario the universe is cyclic; it begins with a Big Bang, pushing everything outwards. At some point, this expansion will stop and go the other way; collapsing back inwards, causing everything to crash together until a singularity is formed - which would then be the singularity for a new Big Bang, a new universe. Like a phoenix rising from the ashes of an older universe. This is a scenario that many people like, unfortunately the universe doesn't care what we like: the current accelerated expansion of the universe seems to disfavour this model. 
• False vacuum. A lot of things in the universe like to be in their 'least energetic state': the configuration that needs less energy. This is known as minimum potential energy principle, and it affects diverse things like electrons in an atom, atoms in a crystal, a marble in a bowl, and those lazy Sunday mornings when we just want to stay in bed and not spend any energy. Going back to the idea of the multiverse, if our universe is just one among billions of expanding bubbles, we could imagine that there are universes out there at a lower energetic state than ours. This would mean our universe is in a 'false vacuum': a local minimum of energy, but not a global one. If our universe came close to one of these other pocket universes, we could 'collapse' to their lower energy level. Imagine if you are heading to the gym but your friends are planning to spend the day sunbathing on a beach. It's quite likely that you would skip your gym plans and adopt their plan of 'least energy'. We can't really fault the universe for doing the same. If this happened, it could fundamentally alter our universe by changing some constants of nature, or even the very nature of space and time. Structures could be destroyed instantaneously, without any forewarning. As scary as that may seem, by studying the particles in our universe physicists believe this couldn't happen for at least a couple of billions of years.

In any case, the Sun will expand in four or five billion years and probably destroy the Earth in the process, so it's unlikely we'll be around for the end of the universe.

But don't worry, we live in a universe with puppies, chocolate, and coffee, so we don't need to concern ourselves with something so many years into the future.

25. How can I become a cosmologist? What good books do you recommend to get me started?
There is still so much we don't know about the universe; we can only see 5% of it, and we probably understand much less. Cosmology is a really active field, with a lot of discoveries waiting for us. If you are considering a career in cosmology, I strongly recommend it. To become a cosmologist first you need a Bachelor/undergrad in Physics, and take as many maths courses as possible. Follow up with a postgraduate in Astrophysics or Cosmology: some countries allow you to directly do a PhD, others require you to first do a Master's. In either case, the postgraduate studies will take about 5-6 years. All in all, to become a Doctor of Physics, you need about 9-10 years of study. It is worth it.
Also, go to as many seminars as possible, try to read scientific articles, and don't be afraid to email the authors if you don't understand things: they are generally happy to discuss their work!

If you want to get involved without all the studying, there are plenty of citizen science projects you can help out with.

Finally, if you want some good books on the topic, check out this list. If you know of any books I forgot to include in the list, let me know!


We did it! We got through the 25 most common questions in cosmology. I will elaborate on most of these topics in the future, but for now we have a good starting point.

Cosmology FAQ Part 3 (The Present)

Some time ago, in a not-too-distant place, I did a 'Cosmology 101' on Periscope, an exciting endeavour in which I tried to answer the 25 most common questions in cosmology. It was long, it was dense, and it was great fun. If you missed it, don't worry, you can read this series of posts, which will cover the same topics.

In Part 1 we covered the Big Bang; the beginning. Part 2 dealt with what's happened since then: 13.8 billion years reduced to one post. Today we will discuss what the universe looks like now.

This series of posts will have a lot of information. If you don't understand everything the first time, don't worry. This is just to give you an idea of the type of topics we can discuss in this blog. If you see anything mentioned here that you would like to know more about, or that you think is unclear, let me know in the comments; that will give me ideas for things to talk about or write about in the future.

So what is the current state of out universe?

9. What is happening to the universe now? 
Ever since the Big Bang the universe has been growing and cooling down. Our evidence shows us the universe is 13.8 billion years old, and the current temperature of the CMB (Cosmic Microwave Background) is T$=2.725$K, which is -270° Celsius, and if you use the weird scale, it's -455° Fahrenheit (seriously though, why do you use this scale?).
Our best model for the universe is the $\Lambda$CDM model. This means the universe is governed by dark energy ($\Lambda$ - pronounced 'lambda'), and cold dark matter. When we say cold dark matter we mean the dark matter is moving slowly - so slow that we don't need to consider relativistic effects. The most recent data tell us the universe contains 5% normal matter, 25% dark matter and 70% dark energy. And that leads us to a very obvious question.

10. What is dark energy, and what is the cosmological constant?
We are not sure what exactly this dark energy - or cosmological constant - is. We are very sure the expansion of the universe is accelerating (I'll come back to this), and we know something must be causing everything to go outwards, despite the attractive pull of gravity, which means it has to have negative pressure. The first person to come up with a valid, and testable, explanation for dark energy could win a Nobel Prize.

The Einstein Field Equations, with $\Lambda$

Historically, the cosmological constant dates back to Einstein. When he formulated general relativity, the universe was thought to be static: neither growing nor shrinking. If this was the case, why didn't the universe collapse due to the gravitational force? Einstein's response was to put a cosmological constant in his equations.

Turns out this guy was right

This fixed the issue, but a few years later the expansion of the universe was discovered, meaning the cosmological constant was no longer needed. Einstein removed it from his equations, and often referred to it as his 'biggest blunder'. In the 1990s it was seen, however, that the expansion of the universe was accelerating. It wasn't just expanding, it was doing so quicker than before. This meant we once again needed some form of 'anti-gravity' in our equations, and so we put the cosmological constant back in. Seems like Einstein was right again.


11. What do we mean by expansion? Are objects moving away from us, or are our definitions of length and time are changing?
By expansion we mean that everything is moving away from everything else; the distance between galaxies is increasing (see question 14).

Case 1: Grid points get further apart

We could consider expansion from two different points of view. Imagine we draw a grid on the universe, and mark galaxies at different points. In our first scenario we fix the galaxies at specific grid points, and these grid points get further apart, causing redshift as they do. If before there was 1 million lightyears between the galaxies, after a certain amount of time there are 1.5 million lightyears.

Case 2: Grid points are fixed

In our second scenario, we keep the grid fixed, and let the galaxies flow freely. As time passes, we notice the galaxies move away from each other, also causing redshift. Like before, the distance between the galaxies has increased (and if my two drawings are correct, you can see that the increase in distance is the same in both scenarios). Both of these points of view are equally valid, and in fact general relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views.

Notice that in the above example, in both scenarios the definitions of length and time are not changing: one small square on the paper is always 0.5 million light years. This is not like when we changed our definition of a planet to kick Pluto out; the definitions of length and time are constant. The distance between two galaxies, however, is not constant: it is gradually increasing.

12. Why do we think that the expansion of the Universe is accelerating?
Imagine you are looking at a 60 watt light bulb. The light bulb always emits the same amount of energy, but if you walk away from it, you notice it gets fainter and fainter. This is because the amount of light you see depends on the distance, even though the amount of light emitted by the bulb is the same. With this idea, you could calculate the distance to the light bulb based on how much light you see. In astronomy we do something similar; we look for objects that emit a well-known amount of light, which we call standard candles. Specifically, we use supernovae: the violent deaths of stars. The amount of energy emitted by a supernova is extremely well know. This means that if we measure how bright the supernova appears to us, we can measure its distance from us, the same as the light bulb.

Redshift and apparent magnitude of supernova. Credit: U. Alberta

The next thing we do with these supernovae is measure their redshift (using their spectral lines), which tells us how much the universe has expanded since the light left the supernova. If the expansion of the Universe is accelerating, the expansion was slower in the past, and the distance to a supernova now would be larger than it would be in a non-accelerating case. If the expansion is decelerating, it was faster in the past and the distance now would be smaller. What we found is that in far away galaxies supernovae appear fainter than expected; therefore they are further away than they would be if they were moving at a constant speed.
If you are travelling in a car at a constant speed of 50 kilometres an hour, after one hour you should be 50 kilometres away. However, if you are 80 kilometres away, it means at some point you increased your speed: you accelerated. The same is true for the universe: if the distant galaxies are further away than expected, they must have accelerated.

13. Can objects move away from us faster than the speed of light?
As always when relativity is involved, the first thing we have to ask is 'from whose point of view?'. Things are relative, after all. Special relativity tells us that two objects cannot pass by each other with relative velocities faster than the speed of light. This is fairly straightforward, the problem comes when we try to apply this to objects very far apart. Then we have to ask who is measuring the distance between the objects, who is measuring their velocity, and if this observer is moving. I'll dedicate a whole post to special relativity in the future. To answer this question, let's assume that the distance to a galaxy is the distance between us at a specific time, measured by a set of observers moving with the expansion of the universe (so in the rest frame of the moving galaxy), and all making their observations when they see the universe as having the same age (the age of the universe also depends on who is looking). In this specific case, velocity of this receding galaxy can definitely be larger than the speed of light. This does not contradict special relativity, because we are not talking about the relative velocity between two objects as they pass each other.

Related to this question, we could ask if the universe is (or was at some point) expanding faster than the speed of light. The short answer is no, the long answer can be found on Sean Carroll's blog.

14. Why doesn't the Solar System expand if the whole Universe is expanding?
The objects in the Solar system are under the constant influence of gravity. Gravity pulls objects together, but it doesn't have effect across large distances. Mathematically, the gravitational force decreases with the distance squared. This means that if we get ten times further away, the gravitational force is a hundred times weaker. Imagine there is a constant battle between the gravitational force pulling objects inwards, and the expansion of the universe pushing objects away. On small scales, such as solar systems (or indeed galaxies), gravity wins, but on large scales the expansion of the universe wins. This also explains why galaxies can collide: on the 'small' scales of galaxies gravity wins, on large scales of the whole universe dark energy wins.

As an analogy, imagine two marathon runners: Bob (gravity) and Lisa (expansion). Bob decides to sprint and try to last as long as possible, while Lisa decides to maintain a calm and steady pace. At the first checkpoint, Bob will be in front of Lisa, but as the race goes on, Bob will lose energy and slow down. By the end of the race, Lisa will likely be winning. On small scales, Bob wins, but on large scales Lisa wins.

15. What is the Universe expanding into?
Nothing! The idea that the universe is expanding into something is a common misconception; we imagine the universe to be in something. I often use the analogy that the expansion of the universe is like a cake in the oven rising. However, the universe is not in anything. The universe is defined as everything in existence: if we were expanding 'into' something, it would imply there is something outside of the universe, but by definition, this would also be part of the universe. This is why we can't talk about 'the edge of the universe'. On a similar note, if the universe has no edges, we say it's infinite (see question 17).

Even if there was something that we are expanding into, it's not something we would have any access to; we have no way of exchanging information with it, so it's not a profitable thing to think about. We are asking about the 'external' geometry of an object in which we are 'trapped'. It's like putting wheels on a tomato: time consuming and completely unnecessary.

16. What is the observable universe? How big is it?
Einstein taught us that the speed of light is not infinite; it has a clearly defined value of 300,000 kilometres per second. The age of the universe is also quite well known. This means that light has only been able to travel a certain distance since the beginning of the universe. This brings us back to the idea of observable universe. The observable universe is everything we are able to see in the universe; it's a sphere centred around each observer. Everyone is the centre of their own observable universe. The light from objects beyond our observable universe hasn't yet had time to reach us.

The size of the observable universe often leads to misconceptions: one could naively assume that as the universe is 13.7 billion years old, the observable universe would have a radius of 13.7 billion light years. This is wrong! The light that we see from a star emitted 5 billion years ago was 5 billion lightyears away when the light was emitted, but the star has moved away from us since then, due to the expansion of the universe. Taking into consideration this expansion, current estimates put the size of the observable universe at 46.5 billion light years in radius, so 93 billion light years in diameter.

Earth's size compared to the observable universe. Image by Andrew Z. Colvin

17. How big is the universe? How can something that started of so small be infinite?
We believe the universe is infinite, but we really have no way of testing what's outside our observable universe. It would take light over 93 billion years to cross the observable universe (even more if we take into consideration expansion), which is a lot. The observable universe might not be infinite, but it is ridiculously big, and we think our observable universe is only a small fraction of the whole thing.

Now we need to make a slight distinction: I've said before that at the Big Bang, everything was contained in a small point. Of course, what we mean is that everything in the observable universe was concentrated into a small point: we can't make the same claim for things outside of the observable universe, at least not without a model to tell us exactly what the universe looked like when gravity and quantum mechanics worked together; for that we need a theory of quantum gravity.
But what we do know is the universe is big. Very big. So big that we can justify saying it's infinite.

18. Is the universe flat? What does that mean?

The universe is homogeneous (same in every direction) and isotropic (same from wherever you look). Mathematically, there are only three possibilities for a universe like this; flat like a sheet of paper, hyperbolic like a Pringle, and spherical like a ball.
This can be seen by drawing triangles: on a flat surface the sum of the angles in any triangle is always 180°. On a negatively curved surface (like a crisp/potato chip) the sum of the angles in a triangle is smaller than 180°. On a positively curved sphere the sum of the angles in a triangle is more than 180°. Our universe is flat; triangles in the universe will look the same as triangles on a sheet of paper.

From a more physical point of view, general relativity explains that mass and energy bend the curvature of spacetime, so the curvature of the universe is related to its density (mass over volume). We use something called the density parameter ($\Omega$), which is defined as the density of the universe divided by the critical energy density: the mass energy needed for a universe to be flat. So our three cases reduce to the following: if $\Omega>1$ the universe looks like sphere, if $\Omega<$ the universe looks like a crisp, and if $\Omega = 1$ the universe is flat. Our measurements show that the density parameter is really close to 1. In fact we find that the universe is flat with only a 0.4% margin of error.


So we've seen that our universe is flat, undergoing accelerated expansion, and filled with a lot of stuff we don't understand. We've covered quite a few questions today; but there is a lot to say about the universe.
In the fourth, and final, part of this series we will discuss the big questions like the fate of the universe.

The UCLA has a nice (but outdated) FAQ about the universe. See it here.

Cosmology FAQ Part 2 (The Middle)

Recently on Periscope I did a 'Cosmology 101', a two-hour long scope where I answered the 25 most common questions in cosmology. You can find a typed up version of those 25 questions and answers, broken down in to smaller chunks, in this series of posts.

There is going to be a lot of information condensed in these posts, so don't worry if you don't understand everything the first time. If you see anything mentioned here that you would like to know more about, or that you think is unclear, let me know in the comments; that will help me see what topics you want to hear about.

In Part 1 I covered the Big Bang; the beginning. In today's post I will discuss what has happened since then. This post is going to use scientific notation; a really useful way of writing long numbers. If you are familiar with how it works, you can jump ahead and ignore this section. If you are unfamiliar with it, this is how it works:

• Big numbers. A number like 'one billion' has a lot of zeros, if we write it all out we get 1,000,000,000. Scientist love saving time, so what we do is we count the zeros, see there are nine of them, and put $10^9$. This means we have a 'one' followed by 'nine zeros'. 
• Small numbers. The principle is the same: 'one billionth' would be 0.000000001. Again, we count the zeros, but this time the zeros are before the one, so we say they are negative, and write $10^{-9}$. So we have a 'one' preceded by 'nine zeros', we add a decimal place after the first zero, and we're done!

7. How old is the Universe? How do we know?
There are two ways we can answer this question. The first involves theoretically calculating the age of the universe, using the Hubble constant (which is a measure of the rate of expansion of the universe) and the Big Bang model. Using the latest data from the Planck satellite, this model-based age is currently 13.82 billion years. This calculation assumes our models are correct. The other way of answering the question is to look at model-independent evidence. Ideally, if our model is correct, the evidence will give the same result as the theory. So what evidence can we use?

• Radioactive decay. The age of chemical elements can be estimated using radioactive decay to determine how old a given mixture of atoms is. We basically look at the composition of a rock, and use the known half-life of elements (the half-life is the time it takes for half of a sample to have decayed) to estimate the age of the rock. When applied to rocks on the surface of the Earth, the oldest rocks are about 3.8 billion years old. When applied to meteorites, the oldest are 4.56 billion years old, which tells us how old the Solar System is. Applying this method to the whole universe is problematic, as we would need to find big old rocks, but we can apply this method to specific elements, like Uranium, which is present in very old, metal poor (earliest) stars. By studying the amount of uranium and thorium (the element uranium decays into) in very old stars, we can put a constraint on the age of the universe. Specifically, physicist found an estimate of $14.5\pm 2.6$, which means it is somewhere between 12 and 17 billion years. 
• Star Clusters. There are millions of stars close to us that we can study. This means we have been able to understand a lot of properties about stars, and make a nice plot known as the 'Hertzsprung-Russell Diagram' to illustrate what we know.

  

  Hertzsprung-Russell Diagram. Credit: ESO

  Most stars belong to the 'main sequence', a family of stars that follow a specific set of rules: their luminosity (amount of light they release) and their temperature are related, the mass of a star is also related to the luminosity, and the mass and the lifetime of the star are also related. This means that the luminosity of a star can give us an approximate age of the star, using some very basic relations. The brightest stars in a cluster are the oldest, and so they give us a lower limit on the age of the cluster. Obviously, the universe has to be older than these clusters. Applying this technique, physicists found that the age of the universe is greater than 12.07 billion years with 95% confidence. Other researches found the mean age of the oldest globular clusters to be 11.5 $\pm$ 1.3 billion years.
• White dwarfs. Imagine a star as massive as the Sun, compressed in a space as small as the Earth. These are called white dwarfs, and they are formed when small stars collapse at the end of their lifetime. These stars no longer experience fusion, so they have no energy source. They still have some residue heat left, but with no energy source, they gradually cool down. This means that the oldest white dwarfs will be the coldest and thus the faintest. We know more or less what temperature the star was when it collapsed (this is known by looking at a lot of stars, and seeing what they do often). If we are able to measure the current temperature of white dwarfs - by measuring their brightness - we can calculate how long they have been burning. Our same stellar models also predict how long the first white dwarfs take to form in the universe. So by searching for faint (old) white dwarfs, and combining this with how old the universe needs to be for them to exist, we can calculate the age of the universe. With this method, the age of the universe is calculated to be $12.8\pm 1.1$ billion years.

Averaging the data from all the methods, we obtain an age for the universe of 13 billion years, with an uncertainty of 0.9 billion years, which is very similar to the age predicted by our theoretical models.

8. What has happened to the universe since the Big Bang?

Quite a lot, actually. The history of the universe is a beautiful topic, but it is quite expansive (like the universe itself!). So much has happened in the universe's history that I decided to dedicate a whole post to it, which I should post in the next few days/weeks. So here I'll just review the key points in the universe's timeline. The main idea to take away is the universe used to be very small and very hot, it has gradually been growing and cooling down, so today it's very cold and very big.

A brief history of time. Credit: ESA - C. Carreau

The earliest time we can conceive of is the Planck time: $10^{-43}$s after the Big Bang. We believe that the four forces (electromagnetic, strong nuclear, weak nuclear, and gravity) were united, but our understanding of this time is very limited. A region about $10^{-33}$cm across is homogeneous and isotropic (it looks the same in every direction wherever you look from), the temperature is T=$10^{32}$K, which is ridiculously hot.

Not long after the Big Bang ($10^{-36}$s), gravity breaks away from the other three forces, and the strong force decides to do the same, but electromagnetism and weak nuclear force are still bound together, like two inseparable friends. When the strong force breaks away, it triggers a period of inflation: rapid accelerated expansion. Everything in the universe shoots away from everything else, extremely quickly. This period of inflation ends about $10^{-30}s$ after the Big Bang. The temperature is still very hot ($10^{27}$K), but due to the quick expansion the homogeneous region is now about 1m across.

Everything we see is made of these little things.

Our understanding of the universe is much clearer from here on. After inflation, the universe is filled with a very hot quark-gluon plasma. As a quick review of the standard model; fermions - quarks and leptons - are particles that occupy space, they are what normal matter is made of. Bosons, on the other hand, are what matter uses to interact. The strong force uses gluons to interact, the electromagnetic force uses photons, and so on. As the universe gradually cools to T$ = 10^{18}$K, the electromagnetic and the weak force finally separate, and the Higgs Mechanism appears. This means that at about $10^{-10}$s after the Big Bang, the universe is governed by the same forces as today.

A millionth of a second after the Big Bang, the temperature is 'low' enough (T$ = 10^{15}$K) for quarks to clump together to form hadrons, known as mesons (two quarks), and baryons (three quarks). For an as-of-yet unknown, but fortunate, reason, there is a small difference between matter (quarks) and antimatter (antiquarks), with 100,000,001 quarks for every 100,000,000 antiquarks and 100,000,000 photons. When the temperature reaches about T$ = 10^{13}$K, quarks and antiquarks annihilate in equal parts, leaving only the small excess of quarks. The homogeneous region of the universe is now at least $10^{18}$m across. Similarly, as the temperature continues to decrease, leptons and antileptons also annihilate into photons, meaning most of the universe is made up of photons (light).

A few minutes after the Big Bang, when the temperature has cooled down to a billion degrees, nucleosynthesis takes place. This means atomic nuclei (atoms without the charged electrons) can be created via nuclear fusion: protons and neutrons fuse together to form hydrogen, helium, lithium, beryllium, and boron. The quantity of these elements is fixed at this stage, so measuring the abundance of these elements in distant galaxies is a great proof of the Big Bang theory.

CMB as seen by Planck. Credit: ESA and the Planck Collaboration

After a busy start, the universe decided to chill out for a bit. Although considering its dramatic start, who can blame it? Not much happened for the next three hundred thousand years or so. The universe got colder and bigger. And then, 380,000 years after the Big Bang, one of the most important events took place: the Cosmic Microwave Background (CMB) is released. When the temperature of the universe has cooled to a couple thousand degrees, protons and electrons combine to form neutral hydrogen. This means the photons are 'left out', and so they start to move around freely, making the universe more transparent. This CMB is everywhere in the universe, and the best part is we can measure this. This is like a window through time: by studying the CMB we can see what the universe looked like nearly fourteen billion years ago. We have put amazing satellites (COBE, WMAP, PLANCK) in orbit to measure the light left over from the Big Bang, to understand where we come from. Isn't that just amazing?

The rest of the history of the universe is less dramatic: 150 million years after the Big Bang, the first stars start to form, with heavier elements forming inside. The stars explode as supernovas, scattering these elements throughout the universe. More stars form, they clump together to form galaxies, and the galaxies clump together to form clusters, which in turn clump together to form super clusters. It's like things suddenly realised the universe was a big scary place, so they decided to all stick together.

Where do we come in to all this? Just over 9 billion years after the Big Bang, so 4.6 billion years ago, the Sun formed. Not long after, the rocks orbiting the Sun formed planets. On the third planet from the Sun, after a few billion years, lifeforms evolved enough to be sat here, reading a blog about cosmology, and probably drinking coffee. Hello, humans!

Only two questions today, but considering we just covered more than 13 billion years, I think we can call it a day. Remember to check out Part 3!

Gravitational waves

"Ladies and gentlemen, we have detected gravitational waves. We did it." Dave Reitze (Executive director of LIGO)

Well that's awesome. What a day, what a moment to be alive. Today it was officially reported that the LIGO collaboration has detected gravitational waves. You've probably seen scientists everywhere shouting with joy. And we have every reason to be happy. 

A few weeks ago, rumours started circulating about a potential discovery of gravitational waves. I really think rumours should be kept out of science, but the good side of those rumours is that they got people talking. I've discussed this topic quite a few times one Periscope, you can watch the most recent one here.

Just to get you started, this video was released a few days before the announcement of detection, and I think it's great. It provides a quick three-minute summary of gravitational waves. This video is like a trailer to my post - you can get an overview, but do stay for all the juicy details.

Now that we've seen an overview, let's try to understand everything going on here.

Massive objects curve spacetime.
Simulation made by C. Joana.

What are gravitational waves?

Einstein's theory of general relativity tells us that space and time are not two separate concepts: they are intrinsically linked - spacetime. You can't have one without the other, they are two sides of the same coin. Furthermore, Einstein's theory says that gravity is just a curvature of this spacetime. Imagine you have a big sheet and you put a ball in the middle: this will curve the sheet (or spacetime), causing other objects to move towards it, following the curvature of spacetime. This means that objects will travel in a straight line on a curved surface. While this might seem strange, it's a phenomenon that we encounter daily: if you take a flight from Los Angeles to Berlin, the plane will pass over Greenland, even though if you look at a flat map this seems like a longer, curved route. If you look at a globe, however, you will see that this is the shortest path (and therefore the straightest) on a curved surface.
Einstein summarised his theory as 'massive objects tell spacetime how to curve; spacetime tells objects how to move'.

This brings us to the idea of gravitational waves.

Gravitational waves are ripples in the fabric of spacetime. Imagine you are in a pool with a friend, and you start dancing in circles around each other. You would notice ripples forming around you and moving outwards. The same thing happens in spacetime. Like all waves, these waves have an amplitude (height of wave), a frequency (how often the crests pass us by), a wavelength (distance between the crests), and a certain speed. The first three are determined by the source of the wave. Therefore, if we are able to measure these characteristics, we could get information about the source. The speed, however, is fixed: gravitational waves move at the speed of light. This should not be surprising as the speed of light is the speed at which 'spacetime talks to itself'.

When Einstein came up with his theory in 1915, he made many predictions: the perihelion precession of Mercury's orbit (the closest point between the Sun and Mercury changes every year), the deflection of light by massive objects (light travels in a straight line on a curved surface), the gravitational redshift of light (light waves are stretched or compressed by gravitational fields), and gravitational waves. The latter was the only prediction from general relativity we hadn't been able to observe. The final proof of Einstein's theory had eluded us. Until now.

What could cause them?
The mathematical formulas from Einstein's general relativity tell us that any massive accelerating object would disrupt spacetime, sending ripples through the universe. So what type of objects accelerate?

• Two objects orbiting each other, like two neutron stars or black holes, will emit gravitational waves.
• Any non-spherical planet or moon; like a planet with a big bump. If it looks more like an American football than a 'rest-of-the-world' football, it would emit gravitational waves.
• A supernova will probably create gravitational waves, unless it explodes in a perfect sphere, symmetrical in every direction.

Just to be clear, we can see some examples of objects that would not create gravitational waves:

• A lonely, non-spinning, solid object, moving at a constant speed and not interacting with the world around it will not generate gravitational waves. This is what we like to call 'conservation of linear momentum'.
• Flat objects, like disks (think galaxies) will not radiate gravitational waves. If you spin a pizza base on your finger (a skill I am yet to master), the pizza will not emit gravitational waves. This is because of something we like to call 'conservation of angular momentum'.

Simulations have existed for years to show us what the merging of two black holes would look like, as well as the gravitational waves they would produce. Like this one, from NASA's Scientific Visualization Studio:

What effects do these waves have? What would they look like?
These small, circular, ripples in spacetime would present themselves as small changes in the distance between two objects. The waves would stretch and compress spacetime; first in one direction, then in the other. If we had particles arranged in a cylinder, this is what we would see:

Here the wave is moving through the cylinder, from back-right to front-left. (A more detailed explanation of this image can be found here.)
How noticeable is this effect? It's really small. Ridiculously small. Even the most powerful gravitational waves (like those that would form in the merging of two black holes) would only change a length by a factor of $10^{-21}$ (zero point twenty zeros and a 1). That's a difference of one thousandth billionth billionth. If this wave went through you, it would change your height by one millionth billionth the width of a single hair.

From a physical point of view, one of the main effects of gravitational wave is that they carry energy away from the source. This means that the waves carry a lot of information about the source objects. In the case of two bodies orbiting around each other, this causes their orbits to decrease: the two objects get closer.

You might be wondering at this point if the Sun-Earth system emits gravitational waves. The answer is yes, it does. This means that the Earth and the Sun are gradually getting closer. Fortunately, gravitational waves become more significant as you increase the mass of the objects, and decrease the distance between them. The Earth and the Sun are really small (on cosmic scales), and quite far apart. The effect of the waves produced in the Earth-Sun system is the Earth gets $10^{-12}$ meters closer to the Sun every year. That means in $10^{12}$ years (a trillion years), we would be one metre closer to the Sun. Considering that is more than the current age of the universe, it's not something we should worry about.

This decrease in orbit has led us to see indirect evidence for gravitational waves before. There is a system of two orbiting stars, known as the Hulse-Taylor system, that we have been able to observe for more than thirty years. Physicists R. Hulse and J. Taylor measured the decay in their orbit: the gradual decrease in the distance between the two stars. This decrease is directly related to the energy carried away by gravitational waves. They were able to show that the decay in the orbit and the decay predicted by Einstein's general relativity were in agreement with less than a 0.2% error. They were awarded the 1993 Nobel Prize in Physics for this indirect detection.

How can we directly detect them?
We can infer their existence from decaying orbits, but we would like to see them directly. So how do we detect something so small and unnoticeable? With lasers!

Recently, Julia Majors PhD (@Feynwoman), everyone's favourite laser physicist, wrote a great article explaining the LIGO set-up and how they used lasers to find these tiny waves. You should definitely read it.

The ideal way to detect gravitational waves would be to measure the small changes in distance between two points. But this presents a problem: a gravitational wave would also make our ruler longer or shorter. We need something that would not change with the gravitational wave. And so we go back to Einstein. His theory of relativity shows us that the speed of light is constant, it would not be affected by gravitational waves. Speed is defined as distance travelled over the time of travel. If the speed of light is constant, changes in the distance of travel would change the time it takes light to cover this distance. All we need to do is use light as a stopwatch.

Enter LIGO, the Laser Interferometer Gravitational-Wave Observatory. LIGO is an experiment currently running in the USA, designed to find gravitational waves using lasers. Like any interferometer (device that measures the interference of light), it relies on constructive and destructive interference.

The red wave is the result of adding the two blue ones together

When two waves overlap, different things can happen. Let's assume the waves have the same frequency and amplitude, as seen in the picture on the right. If the waves meet with the crests overlapping, we have constructive interference, and a bigger wave is created. If, on the other hand, the waves meet with the crests of one wave overlapping with the valleys of the of the other wave, we have destructive interference, and the waves annihilate.

LIGO starts by sending a laser beam to a 'beam splitter', where the light is split in two and sent down LIGO's two arms, each 4 km long. This light is then bounced off of mirrors and sent back to the start, where it is recombined. It is set up so that the beams coming back should produce destructive interference: the light waves cancel out and no signal is produced. However, if a gravitational wave passes through LIGO, it would stretch or contract one of the arms, causing a difference in distance. A difference in distance implies a difference in time: the returning light waves would no longer by synchronised, creating interference that can be measured. If this sounds confusing, don't worry; LIGO made a nice video showing this effect:

Wow, what an incredible system. We use lasers, bounced off of mirrors, to detect minuscule differences in distances, caused by a wave created thousands of miles away. Human ingenuity has no limits.

You might be thinking that such a small difference could be caused by any number of things. What if a train goes past and vibrates the laser? What if there is an earthquake? Wouldn't that create background noise? LIGO has you covered: they made two detectors; one in Louisiana and one in Washington. They are separated by 3000 km, and they serve as a check. A real wave would be detected in both places, with a difference of ten milliseconds (the time it takes the wave to travel that distance), so we can discriminate between real waves and noise. Take that, background noise!

What has LIGO found? 
LIGO had its first run between 2002-2010. They didn't detect anything in that time (not surprising, due to the low sensitivity), so they shut the machine down for five years. In that time, they upgraded everything; better lasers, smoother mirrors, better measuring devices. They switched the Advanced LIGO (we are really not imaginative when it comes to naming our experiments) on in September 2015, with high expectations. In February 2016, they called a press conference, and everyone got excited. Had they finally found the elusive gravitational waves predicted by Einstein a hundred years ago?

Not ones to beat around the bush, LIGO representatives opened their press conference with a dramatic 'We did it.' They found the waves. They had really seen them. This is the official plot they released:

Now we've had a hundred years to prepare for this moment. Physicists have made lists of what interference pattern gravitational waves from different sources would create. And now we have something to compare it to. LIGO compared their signal with all of the predictions (notice the white line in the plots listed as 'prediction'), and found that their signal (received in both detectors, as expected) was compatible with the merging of two black holes. Two black holes, with masses 36 and 29 times the solar mass, merged to create a black hole of 62 times the solar mass. The three missing solar masses were radiated away in the form of gravitational waves. They released a nice simulation showing what they think happened:

Let's just think about this for a moment. 1,3 billion years ago, 12 thousand million million kilometres away, two black holes started a deathly dance, resulting in them merging and sending waves throughout the universe. In September last year, these waves were detected by humans using lasers and mirrors. A hundred years ago, Einstein came up with a way of understanding the universe. He made lots of predictions, and every single one of them has come true. General relativity has stood up to every test we could think of.
That is truly amazing.

For many years mankind has studied the universe. The universe talks to us in different ways, but until now we only listened to one main form of communication: the electromagnetic spectrum. We now have a new way to listen, to uncover the secrets of the universe: gravitational waves. We are the first humans to ever be able to say this. This is a great moment for us.