- 4308 - OBSERVABLE UNIVERSE - and beyond? - But most cosmologists think the Universe is much larger than our observable corner of it. That what we can see is a small part of an unimaginably vast, if not infinite creation. In other words, on a cosmic scale, the Universe is quite small.
----------------- 4308 - OBSERVABLE UNIVERSE - and beyond?
- The most distant
light we can see 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.
-
- Astronomers measure
the “microwave background” because the most distant light has traveled so far
in expanding space it as broadened its wavelength into the microwave
wavelengths.
-
- 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. Since
galaxies cluster at around the same scale throughout the visible universe the
observable universe is “homogenous and isotropic”.
-
- 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 universe implies it is at least
400 times larger than the observable universe.
Inflation would make the CMB temperature uniform.
-
- 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 it, 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 1,026 times larger than the observable universe.
-
- So given all of
this theoretical and observational evidence, how could anyone argue that the
Universe is small?
-
- 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 various models.
-
- Some string theory
models are compatible with “quantum gravity”, and others are not. When you separate the promising theoretical
lands from the “swamp of theories”, what you are left with are theories where
early cosmic inflation isn’t an option. Most of the inflationary string theory
models are in the swampland. This leads one to ask whether you could construct
a model cosmology that matches 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.
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.
-
- Under just the
right conditions string theory could construct a higher-dimensional structure
that matches observation and avoids the swampland. Based on these models, the
Universe may only be a hundred or a thousand times larger than the observed
universe.
-
- 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.
-
-
January 6, 2023 OBSERVABLE
UNIVERSE - and beyond?
4308
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