How large is our Unobservable Universe?

Cosmic inflation began 10 to the minus 36 seconds after the big bang and only lasted 10 to the minus 32 seconds. But in that brief period after the big bang, In that brief flash of time, the universe expanded by a factor of around 100 trillion trillion times its previous size — that’s a one followed by 26 zeroes. but after that, our Universe continued expansion. Now we want to know - more, than 13.7 billion years of expansion, how big is our observable and unobservable universe?

The spatial region that can be observed with telescopes is called the observable Universe, which depends on the location of the observer. The proper distance — the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs) - Making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). (Bars, Itzhak; Terning, John (2018). Extra Dimensions in Space and Time. Springer. pp. 27).

This is our observable universe, but how large is our Unobservable Universe?
How can we see past this horizon? Are we able to find clues within our visible universe to suggest the true size and scale of the bit we can not see? The answer is - yes. We can. From our analysis of multiple sources scientists have concluded, that the universe is almost completely and utterly flat. In fact, the more we analyze the data, the closer to perfectly flat it becomes. with a current estimate of around 0.0002. Our Universe is at least 13.7 Billion years old. We can also measure the curvature of space and the size of the universe.

Based on the flatness we measure, we can confidently say, that the smallest radius of the universe can possibly be (If universe is finite) is a minimum 250 times larger, then the current observable universe's radius. This off course is not its actual size. just the minimum possible size. We can only make inferences based on the laws of physics as we know them, and the things we can measure within our observable Universe. For example, we observe that the Universe is spatially flat on the largest scales: it’s neither positively nor negatively curved, to a precision of 0.25%. If we assume that our current laws of physics are correct, we can set limits on how large, at least, the Universe must be before it curves back on itself. Observations from the Sloan Digital Sky Survey and the Planck satellite are where we get the best data. They tell us that if the Universe does curve back in on itself and close, the part we can see is so indistinguishable from “uncurved” that it much be at least 250 times the radius of the observable part.
This means the unobservable Universe, assuming there’s no topological weirdness, must be at least 23 trillion light years in diameter, and contain a volume of space that’s over 15 million times as large as the volume we can observe. If we’re willing to speculate, however, we can argue quite compellingly that the unobservable Universe should be significantly even bigger than that. (M. Vardanyan, R. Trotta, J. Silk (January 28, 2011). "Applications of Bayesian model averaging to the curvature and size of the Universe". Monthly Notices of the Royal Astronomical Society: Letters. 413 (1): L91–L95.).

Some disputed, estimates for the total size of the universe, if finite, reach as high as 10^{10^{10^{122}}} megaparsecs (Schreiber, Urs (June 6, 2008). "Urban Myths in Contemporary Cosmology").

And yet, perhaps the most jaw-dropping estimate of all is that of an actually infinite universe. our measurements for the curvature of space are now so close to zero, but it really begs a question, could the universe be infinite? If you'd have asked astrophysicists that question in the beginning of the 21'st century, in 2000's, vast majority would have said - No. but in 2020's with all of our evidence the answer really does gravitate towards the terrifying, yet fascinating conclusion - that yes, it really might be.



Will the Universe expand forever?

The fate of the universe is determined by a struggle between the momentum of expansion and the pull of gravity. The rate of expansion is expressed by the Hubble Constant, Ho, while the strength of gravity depends on the density and pressure of the matter in the universe. If the pressure of the matter is low, as is the case with most forms of matter of which we know, then the fate of the universe is governed by the density. If the density of the universe is less than the "critical density", which is proportional to the square of the Hubble constant, then the universe will expand forever. If the density of the universe is greater than the "critical density", then gravity will eventually win and the universe will collapse back on itself, the so called "Big Crunch". However, the results of the WMAP mission and observations of distant supernova have suggested that the expansion of the universe is actually accelerating, which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant. This strange form of matter is also sometimes referred to as "dark energy". If dark energy in fact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.

Infinite Universe?
The density of the universe also determines its geometry. If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere. This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough). If the density of the universe is less than the critical density, then the geometry of space is open (infinite), and negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent.
The simplest version of the inflationary theory, an extension of the Big Bang theory, predicts that the density of the universe is very close to the critical density, and that the geometry of the universe is flat, like a sheet of paper.

Measurements from WMAP
The WMAP spacecraft can measure the basic parameters of the Big Bang theory including the geometry of the universe. If the universe were flat, the brightest microwave background fluctuations (or "spots") would be about one degree across. If the universe were open, the spots would be less than one degree across. If the universe were closed, the brightest spots would be greater than one degree across.

Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe.
(source: WMAP - Shape of the Universe - NASA).

Author: Tornike Pkhaladze