Small universe issue

The universe is big, says Douglas Adams.

The farthest light we can see is the cosmic microwave background (CMB), which took more than 13 billion years to reach us. This marks the edge of the observable universe, and although you might think this means the universe is 26 billion light-years across, thanks to cosmic expansion it is now closer to 46 billion light-years across. By all accounts, this is a very big deal. But most cosmologists believe that the universe is much larger than the angle from which it can be observed. That what we can see is a small part of an unimaginably vast, if not infinite, creation. However, a new study suggests that the observable universe is mostly all that exists.

In other words, on a cosmic level, the universe is very small.

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There are several reasons why cosmologists believe that the universe is large. The first is the distribution of galaxy clusters. If the universe did not extend beyond what we see, more distant galaxies would feel a gravitational pull toward our region of the universe, but not away from us, leading to asymmetric clusters. Because galaxies are clustered at roughly the same scale throughout the observable universe. In other words, the observable universe is homogeneous and isotropic.

The second point is that space-time is flat. If spacetime were not flat, our view of distant galaxies would be distorted, making them appear much larger or smaller than they actually are. Distant galaxies appear slightly larger due to cosmic expansion, but not in a way that suggests an overall curvature of spacetime. Based on the limits of our observations, the flatness of the universe means that it is at least 400 times larger than the observable universe.

Inflation will make the CMB temperature uniform. Credit: Nick Strobel

Then there’s the fact that the cosmic microwave background is an almost perfect blackbody. There are slight fluctuations in its temperature, but it is more uniform than it should be. To explain this, astronomers have proposed a period of massive expansion just after the Big Bang, known as early cosmic inflation. We haven’t observed any direct evidence for this, but the model solves many cosmological problems making it widely accepted. If the model is accurate, the universe is ranked 10th26 Times larger than the observable universe.

So, in light of all this theoretical and observational evidence, how can anyone argue that the universe is small? It’s about string theory and swamps.

Although string theory is often presented as a physical theory, it is actually a collection of mathematical methods. It can be used to develop complex physical models, but it can also be just mathematics in itself. One problem with linking the mathematics of string theory to physical models is that the effects will only be seen in the most extreme situations, and we don’t have enough observational data to rule out different models. However, some models of string theory appear more promising than others. For example, some models are compatible with quantum gravity, others are not. Often, theorists identify a “swamp” of unpromising theories.

Most of string theory is in the swamps. Credit: APS/Alan Stonebraker

When you separate the promising theoretical lands from the swamp, what you’re left with are theories where early cosmic inflation is not an option. Most models of inflationary string theory exist in swamps. This leads one to ask whether you can build a cosmological model that is consistent with observation without early inflation. Which brings us to this new study.

One way to get around early cosmic inflation is to look at higher dimensional structures. Classical general relativity is based on four physical dimensions, three of space and one of time, or 3+1. Mathematically, you can imagine a 3+2 or 4+1 universe, where the global structure can be combined into an efficient 3+1 structure. This is a common approach in string theory because it is not limited to the standard structure of general relativity. The authors show that under the right conditions, you can create a higher-dimensional structure within string theory, which is consistent with observation and avoids quagmires. Based on their game models, the universe may only be a hundred or a thousand times larger than the observable universe. It is still large, but quite small compared to early inflation models.

This is all just speculation, but in a way, this is the case with early cosmic inflation. If early cosmic inflation is true, we should be able to observe its effect through gravitational waves in the fairly near future. If that fails, it might be worth looking more closely at the models of string theory that keep us out of the theoretical quagmire.

reference: Linners, Jean-Luc and Quentin, Jerome. “Little universe.” arXiv preprint arXiv:2309.03272 (2023)

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