Why we can never see the very beginning of the universe
Probably the most interesting and difficult question in the entire history of the existence of reasonable people can be considered the question “where did it all come from?” Some of the most ancient myths are connected with the creation of the world, people and everything else. In different places these myths were different and were told in different ways. It is only relatively recently that there have been glimpses of this question, such as the idea that you can find the answer to it by studying the Universe. Slowly, scientific measurements began to solve the riddles that baffled philosophers, theologians, and thinkers of all stripes.
In the 20th century, the general theory of relativity (GR), quantum physics and the Big Bang theory appeared at once, accompanied by remarkable successes both in observations and in experiments. These platforms allowed us to create theories that we could then test and confirm while rejecting the rest. However, some features – in particular, certain aspects of the Big Bang theory – remained unresolved, and we had to go further, exploring these problems in more depth. As a result, we came to a disappointing conclusion, to which we are accustomed to this day – in the observable part of space there is no longer any information about the very beginning of the Universe. And here’s why it is.
In the 1920s, a hundred years ago, two observations at once changed scientists’ view of the universe forever. A team of astronomers led by Westo Melvin Slifer studied the spectral lines of various stars and nebulae, the features of emission and absorption lines. The atoms of the entire Universe should be the same, and therefore the electronic transitions should also not differ from each other, as well as their emission and absorption spectra. However, some of these nebulae, especially the spiral and elliptical nebulae, had extremely strong redshifts corresponding to high escape velocities – faster than anything else in our galaxy.
Since 1923 Edwin Hubble and Milton Humason began to measure the parameters of individual stars in these nebulae, trying to determine the distance to them. It turned out that they are far beyond the Milky Way – in most cases, these were distances of millions of light years. By combining distances and redshifts, one could get the inevitable output, theoretically confirmed by Einstein’s general relativity: the universe was expanding. The farther away the galaxy was, the faster it moved away from us.
If the universe is now expanding, then it turns out that:
- The density of the universe is constantly decreasing as a fixed amount of matter takes up more and more space.
- The universe is cooling as the light inside it is stretched out, increasing wavelengths.
- Galaxies not bound together by gravity eventually fly apart.
Hubble’s initial observations from 1929, which spoke of the expansion of the universe, were supplemented by more detailed, but still not very accurate observations. However, the graph from Hubble’s work clearly demonstrates the relationship of distance to expansion rate, and contains data that his predecessors did not have. The modern equivalents of this chart go much further. It can be seen that peculiar velocities – the speed of an object relative to a certain coordinate system at rest – exist even at long distances, but the main trend is important here.
These remarkable facts allow us to extrapolate what will happen to the universe with the inexorable passage of time. However, the same laws of physics that allow us to see into the future also allow us to know what happened in the past. If the universe is expanding, cooling, and becoming less dense today, then it was smaller, hotter, and denser in the past.
The Big Bang idea is to extrapolate this process as far back as possible: to even hotter, denser and more homogeneous states. As a result, interesting predictions were obtained:
- more distant galaxies should be smaller in size and mass, their number should be greater, and they should be dominated by hot blue stars;
- the farther in time we look into the past of the Universe, the less heavy elements should be there;
- there must have been a time in the universe when it was too hot for the formation of neutral atoms (and there must have been residual radiation from that time, which has now cooled significantly);
- at some point in the evolution of the universe, atomic nuclei must have been broken by ultrahigh-energy radiation, and only isotopes of hydrogen and helium must have existed.
All these predictions have been confirmed by observations. The residual radiation, which was first called the “primordial fireball”, is now known as the CMB, and was discovered in the mid-1960s. It is considered one of the main proofs of the correctness of the Big Bang theory.
You may think that scientists are able to extrapolate the Big Bang as far into the past as they want, until all the matter and energy of the universe is concentrated in one point. The universe will reach infinite temperatures and density, creating a physical state known as a singularity. In such a state, the laws of physics known to us no longer give meaningful predictions and cannot be considered working.
After a thousand years of searching, we finally got to the origin of the universe. It began with the Big Bang some finite time ago, and corresponding to the formation of space and time. Everything we see is a consequence of this event. For the first time, we have a scientific answer, from which it follows not only that the Universe has a beginning, but also her exact age. As said Georges Lemaitrethe first person to describe the physics of the expanding universe, it was “a day before which there was none”.
But we still had questions arising from this theory, and not finding an answer in it.
Why do different parts of the Universe, not related to each other by causality, have the same temperatures?
Why did the rate of the initial expansion of the Universe (expanding it) and the total amount of energy (fighting the expansion with the help of gravity) turn out to be perfectly balanced in the early stages, accurate to the 50th decimal place?
And why, if such ultra-high temperatures and densities were observed in the past, there are no remnants of all this in today’s Universe?
In the 1970s, the best physicists and astrophysicists struggled with these problems, inventing theoretical explanations for them. Then, in late 1979, a young theorist, Alan Harvey Guth, came up with a brilliant idea that changed the history of the subject.
He put forward cosmic inflation theory, postulating that, perhaps, the idea of the Big Bang allows a good extrapolation of the evolution of the Universe up to a certain point in the past, before which there was a period of inflation, which eventually led to the Big Bang. Instead of arbitrarily high temperatures, densities and energies, the theory of cosmic inflation says that the universe was not filled with matter and radiation. It contained a large amount of energy inherent in the very fabric of space. This caused the universe to expand exponentially at a certain rate, and eventually brought it to a flat, empty and homogeneous state.
And already at the end of inflation, the energy inherent in space itself, and the same everywhere and everywhere, with the exception of quantum fluctuations, turned into matter and energy – and here is the Big Bang.
Quantum fluctuations that occurred during inflation spread throughout the Universe, and at its end became fluctuations in the density of matter. Over time, this led to the formation of the large-scale structure of the Universe, as well as to the temperature fluctuations that are observed today in the CMB pattern.
The move was excellent – it made it possible to convincingly explain the observable properties of the universe that the Big Bang theory could not explain. Causally unrelated parts of the Universe have the same temperature because they all appeared during inflation from one part of space. The expansion rate and energy were balanced because, before the Big Bang, inflation gave the universe the same expansion rate and energy density. And there is no leftover high energy, because the universe only got to its final temperatures after inflation ended.
In turn, inflation produced some new predictions that differed from the Big Bang theory, which did not include inflation. And we could test these predictions by confirming this hypothesis or refuting it. By 2020, we have collected a lot of data that allows us to do this.
According to these predictions, the universe must have an upper, finite limit on the temperature it reached during the hot Big Bang. Quantum fluctuations during inflation turned into fluctuations in the density of the universe, 100% adiabatic (with constant entropy). And these fluctuations should almost, but not ideally, be independent of scale. On larger large scales, their values should be slightly larger than on small ones.
The fluctuations of the relic radiation depend on the initial fluctuations generated by inflation. In particular, the flat part of the graph on a large scale (left) cannot be explained by anything other than inflation. This part represents the germ from which emerged the characteristic pattern of highs and lows that formed in the first 380,000 years of the existence of the universe. And on small scales, on the right side of the graph, the level of anisotropy is only a few percent less than on the left side – on large scales.
Using data from satellites such as COBE, WMAP, and Planck, we tested these assumptions and found that the inflationary theory was consistent with observations. This means that the Big Bang was not the beginning of everything, but only the beginning of the Universe known to us. Before the Big Bang, there was cosmic inflation, which at some point ended and gave rise to a hot Big Bang, and today we can observe traces of this inflation.
But only traces of a tiny interval of this very inflation – perhaps the last 10-32 seconds. Maybe inflation lasted that long. It may have been much longer. It is possible that inflation lasts forever, or maybe it was something fleeting, the product of some other process. It is possible that the Universe was born from a singularity, or appeared as a result of a cyclic process, or has always existed. But there is no information about this in the observable Universe. The nature of inflation leads to the erasure of everything that existed before it.
Inflation is a cosmic “reset”. Whatever existed there before inflation expands so rapidly that in the end there is only an empty homogeneous space with quantum fluctuations. When inflation ends, a tiny amount of this space – from the size of a person to a city block – turns into the observable universe. Everything else, including all the information that would allow us to recreate what happened up to this point, is forever out of reach.
One of the remarkable achievements of science is that we can look back billions of years and understand when and how our universe became the way we see it. But getting answers to these questions, as often happens, only gave rise to new questions. And some of these new mysteries may forever remain unsolved. If this information is not in our universe, some kind of serious revolution will have to happen in order for us to find out where all this came from.