As big as it gets – the large scale structure of the universe
By Chris Fellows, Serenity Mobile Observatory
(click any image to enlarge)
Last time we met we talked about galaxies – the neighborhoods of the cosmos. This time we will be going all the way to the large scale structure of the universe. One interesting “feature” we are going to run into at this level of the universe is Big Al’s (Albert Einstein’s) General Relativity theory.
When I started to research for this installment I immediately ran into a “problem.” How many galaxies are there in the observable universe? Well, that turns out to be a much more interesting question than you might think. If you point a telescope at the cosmos and start counting galaxies you will (eventually) count to about 2 trillion fuzzy blobs out there. If that isn’t mind boggling enough, if you ask some cosmologists how many galaxies there are she might tell you about 200,000,000,000, or 1/10th of what you just counted!
How can this be? Well, the universe is a pretty big thing and light travels at a finite speed – very fast but not infinitely fast. What this means is that when you look into a telescope you are looking into the past: The farther or deeper you look, the longer into the past you are looking. The evolution of the universe, as far as we can tell, went something like this.
The Big Bang happened and space and time came into existence. Picoseconds (one trillionth of a second) later a phenomenon called inflation happened and caused the universe to increase in size by many orders of magnitude, from the size of an atomic nucleus to the size of a grapefruit, almost instantaneously. At this point the four fundamental forces of nature that we know today – electromagnetism, the strong nuclear force, the weak nuclear force, and gravity – separated out from a unified force field and froze into the forms we see today. This began the epoch of matter and allowed for the formation of atoms.
For the next 380,000 years or so the still very hot subatomic particles were too energetic to form atoms and kept the universe opaque to light. But as it continued to expand it also cooled, and hydrogen, helium and a little bit of lithium formed in the caldron of subatomic particles. Then, when the temperature had dropped enough, there was another phase transition and finally light could travel freely through the now transparent universe.
We can still see this last scattering surface in every direction in the sky today. We call it the CMBR (Cosmic Microwave Background Radiation) and about 1% of the static you see on a TV tuned to an empty channel is CMBR radiation. It is as far back as we will ever be able to see with light gathering instruments because there was no light before – the universe was in its dark age.
Now that matter existed, star formation could take place. Tiny perturbations in the initial state of the universe caused density fluctuations in the distribution of matter in a cooling universe. Gravity in the denser regions pulled the gases together and the first stars were born about 300 to 700 million years after the big bang. Let there be light.
These early stars were also not uniformly distributed in the expanding universe: Gravity again will take control and clump stars into groupings we call galaxies. The first galaxies were small, about 1 millionth the mass of the Milky Way, and numerous, about 10 times more of them than we see in the current (local) universe. I think you can now see where this is going.
As time progressed, and the universe continued to expand and cool, small galaxies merged and became larger and larger. This of course means that there are less and less of the earliest galaxies. So, looking as deep (as far back in time) as we can, we still see these first galaxies in our telescopes but they are ghosts. They no longer exist even though we can see them with our own eyes. If you count these, there are 2 trillion galaxies in the observable universe. If you do some math and build a computer model you can estimate that there are “really” about 200 billion galaxies in the universe “today.” Yeah, our language isn’t really up to this task but we do the best we can.
Up to this point we have only talked about baryonic matter, the stuff that everything – stars, planets, you, me – is made of. We now think there is another kind of matter in the universe. It is mysterious, unaffected by light, and therefore not directly observable by our telescopes but makes up over 1/4 of the entire universe. We call it dark matter.
This strange stuff has only one property that we can see: mass. Since it is massive it affects the gravitational field and attracts not only more dark matter but also normal matter like galaxies. We can now “see” the distribution of dark matter at the large scale of the universe is clumpy, stringy, connected, and everywhere. It looks like the structure of a sponge and is dotted with galaxies like lights on a Christmas tree.
So, how big is the universe? Well, we really don’t know because we have a “horizon” we can’t see beyond, so it may be infinite – we simply don’t know. You can ask then, “How big is the observable universe?” Well, that is also complex, but the short answer is about 48 billion light years. In other words – incomprehensibly big.
Here comes the bad news. I said earlier that the baryonic matter, the stuff everything we can see with light, is just a small bit of the overall universe; in fact, it is only about 3% of the matter in the observable universe. I also said that dark matter makes up about 25%. That leaves 72%, nearly 3 quarters, unaccounted for.
The largest part of our universe isn’t matter at all, but energy. Einstein taught us that E=MC2, which means matter and energy are exchangeable, two sides of the same coin. The universe seems to be permeated with energy we can’t see – we call it dark energy – and it is pushing all the physical stuff apart like raisins in an expanding loaf of raisin bread. The dark energy is greater than the gravity that holds everything together – so it is not only expanding, it is also accelerating!
Every day something in the observable universe gets pushed over the cosmic horizon and is forever lost to our understanding. In the far distant future, cosmologists will look into the sky and see only our galaxy. We will be an island in a vast empty ocean of space. We live in a very special time. Future scientists will use their best theories and instruments to look at the sky and come up with the wrong answers. They will know nothing about the vastness or composition of the real cosmos.
I could go on and on about this subject – after all, it is about everything. But I think this has gotten long enough for one article. Since I am back in a dark sky location, next time I think I will give an introduction to space photography – the hobby I love.
Till next time!
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)