There are two prevailing ideas about the Big Bang: that of a singularity that gave rise to everything, and that of cosmic inflation. The first — and most popular — says that all the energy and matter in the universe was confined to a single point of infinite density, while the second dismisses that possibility.
There are some reasons to reject the idea of singularity, such as the very concept of a single point in the universe where everything would be concentrated. As much as it is inevitable in the Theory of General Relativity, the singularity “breaks” mathematics and physics ceases to make sense.
Furthermore, observations of the cosmic background radiation — the remaining light that confirms the Big Bang theory — show that even at the very beginning of the universe there was already a certain configuration, a kind of “fingerprint”, which resulted in mass concentrations and “voids” distributed throughout the cosmos.
Imperfections of the cosmic background radiation
In the first moments after the Big Bang, the universe was hot and dense , formed by a plasma of photons, electrons and baryons. As the universe expanded, the plasma spread out, cooling until electrons could join with protons to form hydrogen and helium atoms.
Only then could electrons travel freely through space, which resulted in the cosmic microwave background radiation (CMB) — an afterglow emitted about 380,000 years after the start of the Big Bang. We cannot observe anything beyond this period, as photons could not travel to us.
Although the CMB is isotropic (has the same physical properties anywhere in the universe), it does show some temperature fluctuations (anisotropy, or imperfections, with some areas hotter than others), which implies differences in energy concentration. And, as Albert Einstein demonstrated with the formula E=mc² , energy is matter—and vice versa.
Thus, such CMB fluctuations correspond to superdense and subdense regions in the cosmos. As the universe expands and cools, the superdense regions will attract more matter and energy, growing over time; meanwhile, the underdense regions will yield matter and energy to the denser ones, becoming empty “bubbles”.
This distribution served as a kind of seed to form the cosmic structures we see today, with clusters of galaxies and the great cosmic web, and any theory about the beginning of the universe needs to explain this phenomenon.
Singularity x cosmic inflation
According to the Big Bang with a singularity explanation, the universe was simply "born" with this set of fluctuations that grew and evolved to result in the gravitational collapse of matter as dense areas drew matter towards them.
As for the theory of cosmic inflation, where the Big Bang appears only after an inflationary period, these imperfections observed in the CMB had already been seeded by quantum fluctuations. The theory that supports this idea is the energy-time uncertainty relation.
These quantum fluctuations are generated at the smallest scales and stretched to larger scales by cosmic inflation, while later and later fluctuations are stretched over the former, creating a superposition of fluctuations at all distance scales.
Ultimately, these superimposed fluctuations spread throughout the cosmos, cause the density imperfections in the early universe, which in turn lead to the temperature fluctuations we measure in the cosmic microwave background. In other words, quantum fluctuations shaped the anisotropy of the universe even before the Big Bang.
The fluctuations on the largest scales could indeed have been created from the fluctuations that arose during the inflationary period before the Big Bang, but there is also another possible explanation: the gravitational growth of structure in the most evolved universe, which has a very distant cosmic horizon. larger than the early universe.
This requires more concrete evidence to strengthen the inflationary model. And they have already been found. One is that if the temperature before a trillionth of a second after the birth of the universe were high enough to allow us to guess the singularity, there would be gravitational waves in the polarization of the cosmic background radiation.
However, in the most recent measurements of the cosmic microwave background this temperature has never been higher 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, so the universe could not be smaller than that in its most primordial stage.
Perhaps the strongest evidence for cosmic inflation is the universe's superhorizon fluctuations: a limit to how far a signal could travel at the speed of light since the beginning of the Big Bang. This boundary is known as the cosmic horizon, while beyond it lies the super-horizon. Below the threshold is the sub-horizontal scale.
When the WMAP satellite observed in 2003 a correlation between polarized light and temperature fluctuations in the cosmic microwave background radiation, scientists gained a powerful tool: temperature fluctuations could finally be measured on each of these horizon, super-horizon scales. and sub-horizon.
Why is that so important? It's just that both models—singularity and cosmic inflation—make predictions about these measurements, and they all match predictions of an inflationary period before the Big Bang.
On the other hand, signals that appear on superhorizon scales shouldn't appear at all if the universe had started with a singularity.
In the image above, the dotted line represents the prediction of the singularity model, while the solid line in waves corresponds to the predictions of cosmic inflation. WMAP observations validated the solid line.
In 2015, the Plank satellite made even more precise observations of superhorizon fluctuations, in a greater number of wavelength ranges, with greater sensitivity to temperature. The final correlation data (below) is even more favorable for cosmic inflation.
This graph, with the observed fluctuations in the super-horizon, demonstrates that the Big Bang singularity model does not match the universe that astronomers observe.
What about the Big Bang theory?
One of the most recurrent explanations of the Big Bang is: if the universe is constantly expanding, we can go back in time until we reach a point where everything boils down to a simple point — the singularity.
However, the graphs above show that we can only go back so far. Prior to that moment, an inflationary state multiplied space in all three dimensions every trillionth of a second, shaping the universe with the quantum fluctuations that determined regions of high and low density.
The multiverse may be inevitable
If the universe existed before the Big Bang and was expanding on unimaginable scales, could other Big Bangs have happened, giving rise to other universes ? Depending on how you observe it, our universe would only have a chance to exist if others were also born.
Our universe is perfectly fine-tuned to support everything we observe — galaxies, stars, planets, life. So far, scientists still cannot say whether this was luck or the result of some other “rule” beyond our understanding.
This rule could well be the probabilistic one: if a large number of Big Bangs had given rise to endless other universes, the tiniest difference in the quantum fluctuations between them could result in some successful universes and other stillborn universes.
According to String Theory (not yet proven observationally), reality consists of about 11 dimensions, 8 of them so compact that we can't see them. Each type of compaction could create a universe with different physical properties, and the laws of probability would cause other Big Bangs to have these differences from ours.
Therefore, other universes may be governed by different laws, with exotic physics, gravity working in other ways, and so on. In fact, recent research suggests that universes even friendlier to life than ours may exist, although most could be born "failures".
Of course, none of this can be proven, but the possibilities of finding evidence are real. But, first of all, scientists will still have work to find more evidence for the cosmic inflation model, until it is, in a way, unquestionable.
Source: The Astrophysical Journal , University of Chicago , ESA ; via: Starts With a Bang
1_Chris Blacke/Sam Moorfield
2_Graph of the expansion of the universe throughout its history (Image: Reproduction/Alex Mittelmann/Coldcreation)
3_Successive improvements in observations of anisotropies, or temperature fluctuations in the cosmic background radiation (Image: Reproduction/NASA)
4_The fluctuations in the cosmic background radiation indicate where the densest regions formed galaxy clusters.
5_Hot, cold, and average temperature regions in the cosmic background radiation correspond to subdense, superdense, or medium density regions at the time it was emitted, 380,000 years after the Big Bang (Image: Reproduction/E. Siegel/Beyond the Galaxy )
6_Results from WMAP observations favor the cosmic inflation model (Image: Reproduction/A. Kogut et al./ApJS, 2003/Notes by E. Siegel)
7_Results of the Plank satellite observations (Image: Reproduction/ESA/Planck Collaboration; notes by E. Siegel)
8_How many universes are there besides ours? (Image: Reproduction/Geralt/Pixabay)
