Perhaps you’ve heard that the Big Bang started with a singularity, that is, a point of infinitesimal size with a density that tends to infinity. Put that way, this concept may be a bit vague, but it does mean that at a singularity the math stops “working.” However, it may be that the universe was not in such an extreme situation when the Big Bang occurred.
finding the uniqueness
To better understand some astrophysicists’ problem with the singularity, we first need to understand how researchers concluded that the universe was once tiny in size. Until Edwin Hubble’s time, the cosmological model was stationary, that is, the universe was considered to be static. In 1929, however, Hubble found that the universe was expanding, an idea that shocked scientists at the time.
So Georges Lemaître, a priest and physicist, suggested that if the expansion of the universe could be projected back in time, it would get smaller. The longer in the past, the smaller the universe, until at some point the entire mass of the universe would be concentrated in a single point, a “primitive atom.” Of course, the idea was not immediately accepted, as the consensus was that matter is eternal.
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It was only in 1948 that George Gamow, a supporter of Lemaître’s proposal, published an article with Ralph Alpher and Robert Herman describing primordial nucleosynthesis, that is, the period of 10 seconds to 20 minutes after the beginning of the universe. Fred Hoyle, who defended another hypothesis, quipped, saying “this big bang idea” to refer to the theory of “rivals” during a BBC radio broadcast. The term “caught” and the Big Bang gradually convinced the scientific community.
Fortunately, in 1965, cosmic microwave background radiation (CMB), an electromagnetic radiation predicted by the trio Gamov, Alpher and Herman, (the same as in primordial nucleosynthesis), was discovered. It is a “fossil” of light, resulting from a time when the universe was hot and dense, 380 thousand years after the Big Bang. The following year, Stephen Hawking delivered his doctoral thesis, proving the idea that the stationary universe is untenable in the face of observations — work that made the young physicist a celebrity.
Hawking also defended the idea of singularity, something that other theoretical physicists have shown is possible through Einstein’s General Relativity equations. But, interestingly, relativity cannot explain the singularity, so it is common for scientists to use expressions such as “physics makes no sense” when referring to this point of infinite density. Even so, many theories in physics have mathematical singularities of one kind or another.
We can say that, generally, the appearance of a singularity in a mathematical theory means some gap, some missing piece. That’s why they’re important—their existence presupposes a flaw in the predictions of general relativity. Both the relativity theory’s description of spacetime and matter cannot be correct near a singularity, so many scientists try to find the missing theory to explain these inconsistencies.
Okay, but what about the Big Bang?
Returning to Lemaître’s reasoning, if we go back in time, the expanding universe we know becomes a shrinking universe. And if we extrapolate this “flashback”, we will reach the moment when everything was concentrated in a single point. This is possible thanks to current knowledge about the universe—for example, the cosmos is isotropic, that is, it has on average the same properties in every direction we look.
Also, it’s homogeneous, which means that, on average, it has the same properties everywhere we could go. In other words, the universe is the same in terms of matter and energy, everywhere and in every direction we look. So we can consider a universe that must expand or contract. If you can determine what’s in the universe, you can use known equations that will tell you everything about the properties of the cosmos in the past and future.
These predictions include the size of the observable universe, the expansion rate of the cosmos, and the amount of energy in each component (radiation, normal matter, dark matter, neutrinos, dark energy, etc.) of the universe—all of which can be calculated on any point of the past or future. Finally, another important element for these predictions is the cosmic background radiation, which tells us about the state of the universe just after the Big Bang.
If the universe were always full of matter or radiation, we could go back in time until we reached a single point of infinite density, infinite temperature, of infinitesimally small size space, at an instant in time that corresponded to “zero”. There would be no limit to how far you could go back in your equations. But if the universe were to emerge from a state like this, there would be observable consequences.
One of the evidences showing a non-singular beginning is in the cosmic background radiation itself, in the form of temperature fluctuations. If everything had started at a point of infinite temperature, these fluctuations would have been as large as the proportion of the maximum energy reached by the Planck scale — somewhere around 10¹⁹ GeV. But astronomers observe much smaller fluctuations than that — about 10¹⁵.
This brings us to a limit to how far we can extrapolate our journey into the past, and this limit says that there must have been a phase that preceded and shaped the Big Bang. This phase found an explanation in the early 1980s, with the hypothesis of cosmic inflation. According to this idea, the universe was once dominated by a large amount of energy (similar to dark energy, but much greater in magnitude) that caused the universe to expand at an exponential rate.
Then, the universe turned cold and empty, except for the energy inherent in the inflationary field. Then, after expanding like this for an indefinite period of time, the inflationary field decayed, converting almost all of its energy into matter and radiation. If so, considering that the hot Big Bang occurred 13.8 billion years ago, how big was the observable universe at the beginning?
Today, because of expansion, the universe is 46.1 billion light years in radius, in all directions, from our point of view. As we see in the graph above, when matter began to dominate radiation, the universe was about 10,000 years old and about 10 million light years in radius. When the cosmos was only about 100,000 light-years across, roughly the size of the Milky Way, it was only about 3 years old.
These milestones are very useful as we now have more information to go back to the “zero” point. When the universe was about 1 year old, it was smaller than today’s Milky Way and it was incredibly hot: about 2 million K, almost enough to start nuclear fusion. When it was just 1 second old, the universe was too hot for nuclear fusion to occur and measured only about 10 light years.
In just a trillionth of a second after the birth of the universe, it measured just the size of the Earth’s orbit around the Sun and the expansion rate was 1029 times greater than it is today. However, there is a limit to how far we can go back in time, because before there was space, there was also no time. And here comes the second important evidence against the singularity: if the temperature before that trillionth of a second were high enough to allow us to assume the singularity, there would be gravitational waves imprinted in the polarization of the cosmic background radiation.
With today’s measurements of cosmic background radiation, physicists can say that the universe didn’t get any hotter at the hottest time of the Big Bang than about 10¹⁵ GeV, in terms of energy. This implies a minimum size limit. For a time of 10⁻³⁵ seconds, we have a scale of approximately 1.5 meters. In other words, if we can assign a “size” to the universe at its most primordial stage, it could not be smaller than a small human being.
This may not be a definitive answer, given that just over a decade ago astronomers could say that the universe began as roughly the size of a soccer ball. This was because the radiation measurements were not yet accurate enough. In any case, as interesting as it may seem to imagine a singularity of infinite density, there is a limit imposed by astronomers’ observations. And, in many cases in astronomy, boundaries are the most important information you can get about anything in the universe.
Source: Starts With a Bang
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