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Cosmology: A Brief History of Our Universe

Carl Sagan’s Cosmic Calendar

Carl Sagan was one of the most brilliant science communicators who ever lived. One of the devices he used to help communicate cosmic time scales is his Cosmic Calendar included here for illustration.

The most amazing thing that this illustrates is how long it took for our Sun to evolve and then our planet and all life on it. Billions and billions of years. As far as we know, the Universe is approximately 13.7 billion years old, plus or minus a few hundred thousand years. And we know this by using Einstein’s famous theory of General Relativity.

Olbers’ Paradox

Have you ever wondered why the night sky is mostly dark, punctuated by starlight? Assume for the moment that the universe is approximately the same in every direction, which is a decent first approximation. So there ought to be stars in every direction that we could point in. But not all stars shine with the same brightness. Indeed, we know that for electromagnetism there is an inverse-square law. Stars that are three times further away are nine times as dim, and so forth. A competing effect is that as you go further away, the surface of the night sky a certain distance away (a sphere) grows with radius like radius-squared. Combining these two facts -- the inverse-square law for star brightness and how spheres grow with radius -- tells us that the night sky should be ablaze with light in every direction. The fact that it isn’t is called Olbers’ Paradox.

Olbers’ conclusion is the correct conclusion if the universe is static, i.e. not moving, and also infinite in size. The solution to Olbers’ Paradox is absolutely mindblowing: our observable universe is actually finite and expanding! So how do astrophysicists know this?

Hubble’s Law

In 1929, Edwin Hubble put together a bunch of data (making use of other physicists’ data, e.g. Slipher) to propose a law saying that the universe is expanding. What led him to propose such a heretical idea? Well, he looked at a spectrographic analysis of starlight. In plain English this means that he looked at the emission spectrum of each star in the data set, and analyzed them all. Why was this so important?

Quantum mechanics tells us the photon energy and frequency are related by $E=hf$. If the photon frequency $f$ gets redshifted compared to what is observed in the atomic rest frame, then by special relativity we know that it is moving away from us and how fast. Hubble found that, on average, the more distant light sources were moving away from us faster. $$ v = H_0\, D \,. $$ This says that the speed $v$ is proportional to the distance $D$ with proportionality constant $H_0$. The constant is known as the Hubble constant.

Physicists realized that the reason why the universe is expanding is that the fabric of spacetime itself is expanding. It can be explained in a similar way to how dots drawn on a balloon that is then blown up move apart from each other faster as the balloon is blown up. Note that, when spacetime expands like this, it does not change the size of gravitationally bound systems like stars with planets or galaxies. It just increases the space in between galaxies and stuff.

[an image denoting, in two frames, the idea of expansion of the fabric of the universe. ]

Cartoon Summary of Our Origins

[a cartoon picture showing the history of the universe with the beginning amusingly labelled as the Flying Spaghetti Monster, deity of the Pastafarian religion.]

Note: the Flying Spaghetti Monster is the deity of the Pastafarian religion. To see more details about this religion (created by a pissed-off science student) visit venganza.org.

Let us now look at each of the epochs of the evolution of the universe since the Big Bang.

Planck Epoch (speculative)

In this incredibly intense phase of the universe, gravity and everything else is wildly quantum mechanical. Things that are extraordinarily unlikely in our current epoch happen all the time up at the Planck scale - all the wildest things you can possibly imagine are all going on at once. Smooth spacetime is utterly impractical, as ambient energy makes any graviton scatter very strongly off any other graviton. Temperatures are so high that forces unify, matter and force merge, quarks and leptons unify... into strings(?). Experimentally we have no direct evidence of what happened in this epoch, and we may never know. Part of the problem is that to tease apart all the aspects of Planck-scale physics we would need to be able to experiment on lots of different types of universes. But as observers living where we do, we only have ONE universe to experiment on to learn things. So we may never know what goes on in the Planck epoch because of the limitations of physics itself.

Inflation (speculative)

In this phase, the whole early universe underwent a super-accelerated expansion. The universe grew stupendously during this inflationary epoch: by a factor of thirty orders of magnitude. Causality is not broken by this extremely fast expansion: there are no observers in the early universe who could see superluminal effects. It is only the fabric of spacetime that grew so fast. The physical utility of inflation is that it explains a number of puzzles about the universe: homogeneity, flatness, lack of monopoles, and the horizon problem. It is not necessary for us to get into the nitty gritty details of each of these, but if you expand a little on these topics in your essay it will impress me. There are actually lots of different inflation models that differ in detail. Most of them rely on the dynamics of a quantum field (or fields) with spin zero known as the inflaton. At the end of inflation, reheating fills the universe with radiation (photons) and matter.

Baryogenesis ($10^{-11}$ seconds)

In the beginning, strings and anti-strings (or particles and anti-particles, according to taste) were as numerous as each other. But once the universe cooled down far enough, matter began to outnumber antimatter. This was a very small fractional difference, but it turned out to be crucial for us to exist! From a theory perspective, the hard thing is to explain the details of how this matter/antimatter asymmetry arose. Explaining baryogenesis requires physics beyond the Standard Model (BSM). The jury is still out regarding which theory is right. The LHC should teach us more about this.

Electroweak Symmetry Breaking ($10^{-10}$ seconds)

Above the electroweak temperature, the electromagnetic force is indistinguishable from the weak nuclear force. The Ws and Z and photon are all massless in the very early universe. Below the electroweak temperature, crystallization upon cooling happened differently for the electromagnetic and weak nuclear forces. The Higgs mechanism only gave mass to the weak vector bosons, and not to the photon. The above figure is a cartoon of how the various forces split off from each other and at what temperature. (Note: I put Calvin in there from Calvin and Hobbes because his cartoonist made up an awesome name for the Big Bang: The Horrendous Space Kablooie.)

[a cartoon picture depicting the big bang, then at temperature about 1E32 degrees Kelvin (K) gravity splits off, then at about 1E30K the strong nuclear force splits off, then at about 1E15K the electroweak force splits into the electomagnetic and weak forces. ]

Colour Confinement ($10^{-4}$ seconds)

Above the confinement temperature, quarks antiquarks and gluons were free to roam and mess about interacting weakly. Below the confinement temperature, all quarks and gluons (every single one of them everywhere!) have to be strongly bound together into colourless hadrons: mesons (quark-antiquark) or baryons (colourless combos of three quarks) or colourless glueballs.

[on the left is depicted a baryon made of three quarks, with red green and blue making white, on the right is depicted a meson made of a quark and an antiquark, with green and magenta (equal parts red and blue) making white. ]

Proton/Neutron Ratio (1 second)

Since protons and neutrons are far heavier than their electroweak cousins (electrons, mus, taus and their respective neutrinos), they get affected first by a lowered temperature. The reason is that when you have a limited energy budget making heavier particles becomes harder to do because each of them is so expensive: it requires mc² of energy which might not be easily available. Now, at low temperature, protons are stable, even outside the nucleus. Free neutrons, on the other hand, are not stable: they decay in an average of a bit under 15 minutes. Free neutrons (i.e., those not bound up stably in atomic nuclei) decay into a proton, an electron, and an electron antineutrino. What this implies is that turning a proton into a (heavier) neutron requires energy while the reverse process does not. The lack of symmetry in this regard resulted in a p/n ratio of about 7.

Big Bang Nucleosynthesis (100 seconds)

Nuclear forces are short-range. The proton and neutron won't stick together to make deuterium (or anything else) unless they can get really close. At higher temperatures they simply race around too quickly to form nuclei. But after the universe has become cool enough, nucleons (protons and neutrons) do get close enough to bind, and they can then coexist inside the nucleus. The details of all this are complicated technically but conceptually straightforward. During BBN, no heavy elements were made. Only H, a bit of He, and a teeny bit of Li got made in this epoch. Then where, you might ask, do all the heavier elements come from, like carbon C, nitrogen N, oxygen O, and phosphorus P, which we need for carbon-based biological life forms? They were actually created in stars -- in the process of fusion in both small and large stars and in dying-star explosions known as supernovae. We are, literally, star stuff. In particular, every one of the molecules of your body is made of star stuff. We all are. Isn’t that amazing and wonderful and deep and connecting?

Matter Domination (70,000 years)

Radiation dilutes fast as the universe expands. Matter doesn’t get diluted by expansion of the fabric of spacetime as quickly, because some of its energy is locked up in the rest energy. Radiation gets so diluted that by about 70,000 years photons interact with electrons/positrons much less frequently than they interact with other photons. In other words, the photons have thermalized. They start behaving like radiation that comes from a blackbody, an almost perfect radiator. In fact, the radiation left behind after the Big Bang -- known as the Cosmic Microwave Background Radiation (CMBR) -- is the most perfect blackbody known to humankind. It has a temperature currently of about three degrees Kelvin above absolute zero. This is in the microwave part of the spectrum, near the border with infrared.

Recombination (370,000 years)

Finally the universe got cool enough that electrons could happily fall into orbit around nuclei. The universe is electrically neutral on average (unlike gravity, which has no analog of a negative charge). After about half a million years after the Big Bang, any remaining photons can go about their business sailing about the universe without electromagnetic interference from electrons/positrons. These are the photons that astronomers catch in their various telescopes, ground-based and satellite-based. Now, if we look further back in time, we are seeking older light. The rub is that faraway sources are fainter. Star Formation (1,000,000,000 years) Gravity forces, while weak between any two subatomic particles, can get strong enough for almost anything when you get enough energy/mass together. If you wait long enough, the energy locked up in matter (mostly in hydrogen) sources gravitational attractions, which help clump the matter. Eventually, dense enough regions form to allow the first stars to ignite -- via nuclear fusion.

[the Milky Way, based on images obtained by the European Southern Observatory]

The above is a composite picture of the Milky Way galaxy produced by the ESO (European Southern Observatory). Isn’t it beautiful? I don’t know about you, but I find cosmology at least as beautiful as the most amazing artworks I’ve ever seen.