“It took less than an hour to make the atoms, a few hundred million years to make the stars and planets, but five billion years to make man!” -George Gamow
Let’s pretend that, for all of our history on Earth, we had never once bothered to look up with any instruments beyond what our own eyes could offer. Imagine that all the technology we’d have would be the same — telescopes, electronics, GPS, etc. — as would our fundamental scientific knowledge — Einstein’s General Relativity, the Standard Model of Particle Physics, etc. — but we had just never bothered to turn our attentions toward the Universe beyond our sphere of Earthly concern. (I know, I know, you can’t even imagine. But imagine!)
What would we find, today, if we turned our attention upwards for the first time ever?
Up in the night sky, we’d find some different classes of objects. Some wouldn’t twinkle, ever, under any atmospheric conditions. These objects — the Moon, satellites, and the planets — we could easily see, with a telescope, had large angular sizes and big, identifiable parallaxes, allowing us to determine their actual size and their distance from us. These would be the objects within our own Solar System.
There would also be stars, in a variety of colors, temperatures, sizes, and distances. We would quickly discover the relationship between the distance to a star and its apparent brightness, and how that was related to its intrinsic brightness.
We’d slowly start to — with lots of observing targets and time — learn the science of astronomy. We’d learn about different star types, including main-sequence stars, red giants, variable stars like Cepheids and RR Lyrae stars, and stars that went nova or even supernova!
Armed with the knowledge of what’s in our Solar System and of the stars that lie beyond it, we’d have a strong base for peering beyond, into the rest of the Universe. So that finally, when we looked up at the third type of object in the night sky — the extended nebulae — we’d be ready to learn lots of interesting things about them.
How old are these young star clusters? With an understanding of stars, we can tell you. How far away are these nebulae and supernova remnants? By understanding individual stars and the distance/brightness relationship, we can tell you. And finally, what about these faint, fuzzy blobs and spirals in the sky? Just what are they?
At this point in time, we can resolve individual stars inside many of them, and find that unlike the stars in our own galaxy — which are hundreds, thousands, or even tens of thousands of light years away — these objects are millions of light-years distant. In other words, they are island Universes, or galaxies entirely separate from our own!
This might seem like the most obvious thing in the world today, but consider that this was not known until less than a century ago. And while you were making these measurements of distances to these galaxies, you might have noticed something else.
These galaxies were not just very distant, but the light coming from them was also redshifted. When objects move towards or away from you, the frequency of the light gets shifted towards the blue or red end (respectively) of the spectrum, with the faster motions corresponding to a swifter velocity. According to general relativity, the expansion (or contraction) of spacetime could cause the same type of red (or blue) shift of the light.
What you’d find, when you looked out at all of the galaxies you could see, would’ve been something remarkable.
You’d find that the more distant a galaxy was from you, on average, the more redshifted its light was! You’d notice that this was virtually independent of direction on the sky, and that — excepting the fact that there was a “scatter” of a few hundred to maybe a thousand km/s — this was a Universal relation, extending for not just millions but billions of light years!
From this alone, you could draw a few different conclusions depending on how you interpreted your data, such as:
- the Universe was such that we were at the center, at rest, and that objects were moving away from us, with further objects moving away faster,
- light was getting tired, and that the further away a light-emitting object was, the more energy it lost, shifting further into the red end of the spectrum, or
- the Universe was expanding under the rules of General Relativity, and that the galaxies’ light shifted deep into the red because of the Universe’s expansion.
If this last option were true, we’d have a very interesting picture of the Universe’s history.
We’d have a Universe that was expanding, that was smaller, denser, and (because of how wavelengths/frequencies work) hotter in the past. Which means we’d have a Universe that was expanding, diluting, and cooling today.
This “model” of the Universe is one you might recognize: this is the Big Bang picture of the Universe! If this were true, you’d ask yourself, what else would we expect to be the case?
If we looked into the past, we’d expect that the Universe would have been more uniform, with fewer large galaxies and fewer giant clusters of galaxies. After all, if the Universe has been around for a finite amount of time, and gravity attracts things over time, the structure that existed billions and billions of years in the past should consist of smaller galaxies that are less clumped together than the ones that exist today.
In other words, the Universe should have been more homogeneous in the past. We also said that the Universe should have been hotter in the past! What does that mean?
It means, at some point, the average temperature/energy of a photon in the Universe should have been so high that neutral atoms — the stuff that makes up everything we know on Earth — would not have been able to form! A hot, ionized plasma is all that should have been around, as every time an atomic nucleus tried to capture an electron, a photon should have come along and blasted it apart. So at some point, the Universe should have been filled with a hot, dense plasma. (Which we know — by the way — is opaque, or not transparent, to light! Remember this!!!)
But we can go back even further! Imagine a time that was even hotter and denser than when this plasma existed, to a time where it was so hot that even protons and neutrons — the constituents of atomic nuclei — would be blasted apart by the scorching hot radiation of the Universe!
At some point, the lightest elements in the Universe would have been unable to form. These are some of the consequences of this Big Bang model of the Universe, and these are theoretical predictions that we can test!
Each of these events will leave observable signatures behind. If we start out in a hot, dense, roughly uniform state and come forward in time, we can predict what we should see today based on the Big Bang model of the Universe! Let’s start at the beginning and come forward.
The light elements: as the Universe expands and cools from an incredibly hot, dense state, eventually it will cool enough that the protons and neutrons, left over from an even hotter, denser state, will fuse together into the light elements deuterium, tritium, helium-3, helium-4, lithium-6, lithium-7, and beryllium-7. The only parameters that determine how much of these light elements get created are the ratio of photons to protons+neutrons. Because we know the particle physics behind it, we can know how much helium-4, helium-3, deuterium, lithium, etc., should be left over from the Big Bang, dependent only on that one, measurable parameter. If we can find some pristine gas from the early Universe, all of these elements should exist in those predicted abundances.
The leftover radiation from the Big Bang: better known as the Cosmic Microwave Background! Because the hot plasma was opaque to light, we can’t see all this radiation from the Big Bang until these neutral atoms form. But once these neutral atoms form, that leftover radiation from the Big Bang should not only stream directly to us, it should come to us practically uniformly in all directions, with a predictable, blackbody spectrum stretched by the expansion of the Universe. (Note that the other, above explanations for redshift — including tired light — do not give the proper spectrum!)
The discovery of this leftover radiation and the accurate measurement of its spectrum led, historically, to the acceptance of the Big Bang, as no other model of the Universe explains this observation, the abundance of the light elements, and the redshifts of the distant galaxies simultaneously. But there is one more great observation we can make.
The Large-Scale Structure of the Universe: from the earliest stars and galaxies to modern times, from isolated dwarf galaxies to humongous clusters and superclusters, some of which have behemoth galaxies maybe 100 times the mass of the Milky Way inside of them, we should find larger, clumpier structure in the Universe today and more sparse, uniform structure in the past.
And we do! To all of it: WE DO!
And that’s what the Big Bang is. That’s how we’d figure it out today, and that’s how we figured it out historically. And — this is important, detractors and skeptics — it isn’t everything.
It doesn’t tell you exactly how much structure you have in the Universe and on what scales; you need a set of initial fluctuations for that, and that’s what inflation gives you. It doesn’t tell you exactly how the Universe has expanded over its history; you need to know how much total matter and dark energy are in the Universe for that, which is something the Big Bang doesn’t predict for you. (You might assume that there isn’t any dark energy, and that all the matter is normal — protons, neutrons, and electrons — but that would be awfully presumptive of you!) It doesn’t tell you how the structure the Universe contains evolves over time; you need dark matter in addition to normal matter to get that right. And it doesn’t tell you about the pattern of fluctuations you should see in the nearly-perfectly-uniform microwave background: you need inflation, dark matter, and dark energy for that. (Incidentally, the same amounts and types that the other measurements told you that you’d need, but that’s a story for another time!)
But you mustn’t deny the Big Bang because it couldn’t predict those things. Those things went beyond the scope of the Big Bang. The Big Bang knows what to do with them if you put them in, but just like any theory, it can’t do everything by itself. But that’s what the Big Bang is, that’s how it works, and that’s how we know it’s right.