- 4155 - SIZE OF THE UNIVERSE? Most cosmologists think the Universe is much larger than our “observable universe”. That what we can see is a small part of an unimaginably vast, if not infinite universe. I have trouble with infinite! It goes past my imagination.
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4155 - SIZE OF THE UNIVERSE?
- The most distant
light we can see is in the microwave frequencies and must be down converted so
our eyes can record it. This most
distant light is the “cosmic microwave background” (CMB), which has taken more
than 13 billion years to reach us. This marks the edge of the observable
universe, and while you might think that means the Universe is 26 billion
light-years across, thanks to cosmic expansion it is now closer to 46 billion
light-years across. The universe was
expanding during th time it took the light to reach us!
-
- There are several reasons why cosmologists
think the Universe is large. One is the distribution of galaxy clusters. If the
Universe didn’t extend beyond what we see, the most distant galaxies would feel
a gravitational pull toward our region of the cosmos, but not away from us,
leading to asymmetrical clustering. Galaxies cluster at around the same scale
throughout the visible universe. In other words, the observable universe is
homogenous and isotropic, the same everywhere in all directions.
-
- A second point is that spacetime is “flat”.
If spacetime weren’t flat, our view of distant galaxies would be distorted,
making them appear much larger or smaller than they actually are. Distant
galaxies do appear slightly larger due to cosmic expansion, but not in a way
that implies an overall curvature to spacetime. Based on the limits of our
observations, the flatness of the cosmos implies it is at least 400 times
larger than the observable universe.
- Then there is the fact that the cosmic
microwave background is almost a perfect “blackbody”. There are small
fluctuations in its temperature, but it is much more uniform than it should be.
To account for this, astronomers have proposed a period of tremendous expansion
just after the Big Bang, known as early “cosmic inflation”. -
-
- We have not observed any direct evidence of
“cosmic inflation”, but the model solves so many cosmological problems that
it’s widely accepted. If the model is accurate, then the Universe is on the
order of 10^26 times larger than the observable universe. That is a one followed by 26 zeros.
-
- Although string theory is often presented
as a physical theory, it’s actually a collection of mathematical methods. It
can be used in the development of complex physical models, but it can also just
be mathematics for its own sake.
-
- One of the problems with connecting the
mathematics of string theory to physical models is that the effects would only
be seen in the most extreme situations, and we don’t have enough observational
data to rule out these various models.
-
- Could we construct a model cosmology that
matches observation without early inflation.
One way to get around early cosmic inflation is to look at
higher-dimensional structures. Classic general relativity relies upon four
physical dimensions, three of space and one of time, or 3+1. Mathematically you
could imagine a 3+2 universe or 4+1, where the global structure can be embedded
into an effective 3+1 structure.
-
- This is a common approach in string theory
since it isn’t limited to the standard structure of general relativity. You could construct a higher-dimensional
structure within string theory that matches observation. Based on their models,
the Universe may only be a hundred or a thousand times larger than the observed
universe. Still big, but downright tiny when compared to the early inflation
models.
-
- All of this is speculative, but in a way so
is early cosmic inflation. If early cosmic inflation is true, we should be able
to observe its effect through gravitational waves in the somewhat near future.
If that fails, it might be worth looking more closely at string theory models
that keep us out of the theoretical swamp.
Astronomy is a never ending science!
-
-
September 16, 2023 SIZE OF THE
UNIVERSE? 4155
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