One of the biggest questions about our universe is where it all came from. When we discovered that the giant spirals in the skies were galaxies not much different from our own Milky Way, we first began to understand the magnitude of what we perceive. These distant “islands of the universe” are not in the Milky Way: they are collections of billions or trillions of stars, separated by millions or billions of light-years in space.
When we discovered that the farther a galaxy is from us, the faster it leaves our perspective, a curious thing opened up before us, which is consistent with general relativity: perhaps it is not galaxies that are moving away from our location, but the fabric of space itself is expanding. If so, then the universe must not only expand, but also cool, and the wavelength of light must stretch to lower and lower energies over time. In addition, we can extrapolate this not only forward but also backward: at a time when the universe was smaller.
Looking in this direction, we see that the universe was denser, hotter, expanded faster, and was more compact. In his earliest youth, the universe was so energetic that neutral atoms were torn apart, and even before that they could not form even individual atomic nuclei.
Such a picture - the Big Bang - was confirmed by the discovery of the relic radiation, the cosmic microwave background, measurements of its spectrum and fluctuations, as well as the discovery of the primary elements that have remained since then. But as tempting as it may be to go all the way back to an extremely hot and dense state, to a singularity, it is simply impossible in our universe.
You see, there are some serious problems that arise if you try to go all the way back this far:
- The universe would not expand indefinitely, would not collapse immediately, would not allow stars or galaxies to form, if the initial expansion rate and energy density were not perfectly balanced.
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- The universe would have different temperatures in different directions - which we do not observe - if something did not lead to a uniform temperature distribution.
- The universe would be filled with high-energy relics that have never been found, as a result of arbitrary extrapolation back to the past.
And again, when we observe the universe, we see stars and galaxies; she has the same temperature in all directions; no high-energy relics are visible.
The solution to these problems was the theory of cosmic inflation, which replaced the idea of a singularity with a period of exponential expansion of space and which prescribed such initial conditions that there could not be a Big Bang. In addition, inflation has made six predictions of what we should observe in our universe:
- Perfectly flat universe.
- A universe with fluctuations on a scale larger than light could overcome.
- A universe with a maximum temperature that will not be arbitrarily high.
- the Universe, the fluctuations of which were adiabatic, or equal entropy everywhere.
- The Universe, the spectrum of fluctuations of which was slightly less than the scale-invariant nature (n_s <1).
- Finally, the Universe with a certain spectrum of gravitational wave fluctuations.
The first one has been confirmed, the sixth is still being sought.
The next logical question about our origins will be, of course, where did inflation come from? Was this state eternal relative to the past (that is, it had no origin and always existed) until the end and creation of the Big Bang? Did this state have a beginning when it emerged from the non-inflationary state of spacetime some specific time in the past? Or was it in a cyclical state when time was locked in a loop?
The difficult thing about this is that there is nothing that we could observe in our Universe, which allowed us to choose one of these three options. In all but the most far-fetched inflation models (and other than the ones we've excluded), our universe has only been affected by the last 10 (-33) seconds of inflation or so. The exponential nature of inflation erases any information that was born before it, separating it from everything that we can observe, blowing it out of our observable universe.
But what remains for us in the form of the observable Universe is enormous: 46 billion light years in radius, 1012 galaxies, 1024 stars, 1080 atoms and about 1090 photons. But these numbers, while astronomical, are finite and don't give us any information about what happened in the universe prior to this tiny last fraction of a second of inflation. We can do theoretical calculations to try to squeeze out some more assumptions, but they will all depend on the chosen model. With the exception of a few specific models that would leave observable footprints in our universe (most not), we have no way of knowing how - or even if - the universe got its start.
The total amount of information available to us in the Universe is limited, and with it the amount of knowledge that we can get about it. However, there is still a lot to be learned, there is still much science does not know. But some things we most likely will never know. The universe may be infinite, but our knowledge of it will never be.