Theoretical models, supplemented by observations made in recent years, now allow cosmologists to constrain the size of the observable Universe relatively faithfully. Beyond this limit, the Universe continues to expand, but remains inaccessible to our instruments. Despite this unknown, it is possible, from the extrapolation of the data collected so far, to make certain assumptions about the size of the entire Universe.
The Big Bang appeared about 13.8 billion years ago. The Universe was then extremely dense and hot, composed of matter, antimatter and radiation. Under the effect of inflation and then expansion, its temperature decreased while its volume increased considerably. Today, the observable Universe has a radius of about 46 billion light-years (or a diameter of 92 billion light-years).
Same topic:
What is the Universe expanding into?
This limit of the observable Universe corresponds to the cosmological horizon, that is to say the border beyond which the Universe and the objects which are there are no longer causally linked to us and therefore remain forever. inaccessible to our observations. This distance is also the last scattering surface — the region of the Universe from which the cosmic microwave background photons were emitted.
To speculate on the size of the entire Universe, it is necessary to rely on information collected within the observable Universe, in particular concerning its history and its dynamics. If today the Universe that we observe is cold and studded with complex structures, in the past, it was much warmer and uniform, not having had enough time to form the large cosmic structures.
Under the effect of the expansion, the wavelength of the photons is stretched (redshift ), corresponding to a decrease in energy, and therefore leading to a gradual decrease in the temperature of the Universe. This means that the young Universe was then hotter, the wavelength of the photons having undergone only very little expansion. A fact confirmed by different distant observations.
It is possible to measure the current temperature of the observable Universe by studying the cosmic microwave background emitted 380,000 years after the Big Bang. These photons are today in the microwave frequency of the electromagnetic spectrum. Demonstrating the properties of a black body, its temperature rises to -270.425°C, corresponding with extreme precision to the predictions made within the framework of the Big Bang model.
In addition, the evolution of the energy of this radiation according to the dynamics of the Universe is known. The energy of a photon is inversely proportional to its wavelength. When the Universe was half its current size, the energy of photons emitted after the Big Bang was doubled. When the Universe was 10% of its current size, the photon energy was multiplied by 10. And when the Universe was 0.092% of its current size, it was 1089 times hotter than today, or about 2700 °C.
To estimate the size of the observable Universe, three parameters must be considered:the current expansion rate of the Universe (obtained by measuring Planck's constant); the current temperature of the Universe (obtained from the cosmic microwave background); the composition of the Universe (matter, antimatter, radiation, neutrinos, dark matter, dark energy, etc).
Thanks to this information, it is possible to determine the evolution of the size of the observable Universe since the Big Bang. Combined with observations of supernovae, large structures and the acoustic oscillation of baryons, the radius of the observable Universe is estimated at 46.1 billion light-years.
The issuance of hypotheses on the entire Universe can only be based on the cosmological model and the physical laws that we know in the observable Universe, and assume that these laws apply both locally and globally.
For example, the latest observation missions have shown that the Universe is spatially flat over long distances with a margin of error of only 0.4%. Based on the standard cosmological model, it is possible to determine a limit size of the Universe before it presents a curvature.
The Planck mission and the Sloan Digital Sky Survey have shown that if the Universe is curved, it has a radius of curvature 250 times greater than what we observe. Assuming the absence of topological anomalies, this leads to a diameter of the entire Universe of at least 23 trillion light-years, containing a volume of space 15 million times greater than that of the Universe. observable.
The Universe went through a period of inflation that eventually came to a halt in our region of the Universe. However, several crucial questions still remain unanswered:what is the final size of the Universe after inflation? Is the hypothesis of eternal inflation correct? Exactly how long did the inflation last? Only the answers to these questions would make it possible to decide clearly on the total size of the Universe.
It is possible that the entire Universe, after inflation, reached a size barely larger than that of the observable Universe. But it is also possible that its size is disproportionately larger. If the model of eternal inflation is correct, then several areas of the Universe have gone into inflation and have formed "Universe bubbles" contained within a global space-time representing the Universe as a whole.
But unless the inflation lasted an infinity of time, or the Universe itself was born infinitely large, it should have a finite size.
