Cosmology – What’s That All About?

So, Cosmology is my current field of study (along with Astrophysics) but I think that it is somewhat of a mysterious subject to most. What do Cosmologists do? What is involved in Cosmology? What is the point?  Well hopefully I shall try to shed some of the mystery…..

What IS Cosmology?

So, the Oxford English Dictionary defines “Cosmology” as;

“The science of the origin, and development of the universe.”[i]

So, what we are studying in Cosmology, is the Universe as a whole. Where it came from, and where is it going? We are looking at trying to constrain the fundamental parameters that govern the universe (the Matter, Radiation and Dark Matter energy densities) and to try to figure out how the universe is moving.

Space, its big, its dark, and its fast…

So, as we all know, space is big. I mean REALLY big! Our current best estimates of just how big are in the order of 93 billion light years ( 1Ly is the distance travelled by light (at ) per year) This is known as the observable horizon. It is likely that in actual fact, space is far larger than this, but the observable horizon as the name suggests is the limit of where we can observe with current techniques.


Figure 1 CMB Map from PLANK

The CMB (Cosmic Microwave Background) actually is the current edge of the observable universe. This is what is known as the surface of last scattering and is a baby picture of the universe.

Now, those of you who are familiar with astronomy are likely thinking “now hold on there one minute, the universe is only 13.8 billion years old, how can it be 93 billion light years across?” Well, that is an interesting question in itself. The answer is deceptively simple. Light travels at a fixed speed (as mentioned above, and as stipulated by Einstein’s Special Relativity) BUT space itself has no limit on how fast it can expand, that’s why we have 93 billion light-years worth of room, in only 13.8 billion years.

1 Cup Dark Energy, 1 Cup Matter, bake for 13 billion years…

So, as I mentioned before, there are 4 main “parameters” which Cosmologists spend their time trying to constrain. The Energy Density Parameters of Matter, Radiation and Dark Energy. The fourth parameter is what is known as the “Hubble Parameter” named after the famous (and rather crotchety Edwin Hubble) which is a measure of the rate of expansion of the universe. This Hubble parameter is derived from the density parameters.

What the Hubble parameter tells us, is that the universe is expanding. Not only is it expanding, but it is actually accelerating too! The current, most popular explanation for this is due to the mysterious dark energy (not to be confused with dark matter, as they share nothing in common, but the word dark in their name) So, where is this taking our universe?

A Date with Fate…

In an expanding, accelerating universe, it has been postulated that a possible outcome is, what is whimsically known as the “Big Tear” where the universe literally expands to a point where it rips apart. There is no need to worry about this impending fate however, this will happen long after the universe has cooled to freezing, and all the atoms have split apart due to the expanse of space.

I’m an Astronomer, why do I care about Cosmology?…

Cosmology has many far reaching implications for the astronomer. Too many in fact to go into in great detail. However, the most well know is Redshift. This is the light equivalent of the speeding ambulance. As it approaches its pitch increases, and as it passes, the pitch decreases, (relative to you standing at the side of the road.) For light, there is no change in pitch, there is a change in frequency. An object moving away from us, will appear red, and an object moving toward us, blue.  This is a very key feature in astronomy, as it enables us to calculate the speed at which objects (like the Andromeda galaxy for instance) is moving away from us. It also gives us a way to give create a cosmological time scale.

What’s the Date, Mr Hubble?…

The furthest away galaxies we can observe with current technology, are at redshifts (z) of around 12. The surface of last scattering (the baby picture of the universe) is at z=1000. So when we are looking back, we are not only looking back in time, but we are also looking back at a time when the universe was very much smaller than it is now. As the galaxies move away from us, the light emitted by them is stretched (due to the stretching of space-time) which changes the frequency. Edwin Hubble was one of the first astronomers (not definitively the first however) to actually recognise that the universe WAS expanding. (It was widely accepted pre 1920 that the universe was static. To such a point that Albert Einstein when fiddling with his equations actually generated a fiddle factor to cancel out the expansion he predicted. He was later quoted as saying “it was the greatest blunder of my life” but there isn’t actually any proof he did say this.)

This fiddle factor that Einstein included in his equation he named “The Cosmological Constant” and as it happens we know it today, as the Dark Energy Density Parameter.

So, we have come full circle. From the birth of the universe, some 13.8 billion years ago, through the expanding universe. We have been red shifted, blue shifted, gravitationally red shifted, and spotted by Hubble moving away from us, and ended up in a chance meeting with Einstein.


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