- 4270 - OBSERVABLE UNIVERSE - how far back? 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” 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.
--------------- 4270 - OBSERVABLE UNIVERSE - how far back?
- 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.
However, a some astronomers argue that the observable universe is mostly all
there is.
-
- 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,
therefore the observable universe is
“homogenous and isotropic”.
-
- 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 this, 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.
-
- Given all of this
theoretical and observational evidence, how could anyone argue that the
Universe is small? It has to do with string theory and the swamp lands.
-
- 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. However, some
string theory models appear much more promising than others. Some models are compatible with “quantum
gravity”, and others are not.
-
- 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.
-
- Under just the
right conditions, you could construct a higher-dimensional structure within
string theory that matches observation and avoids the swampland. Based on their
“toy 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
pretty 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.
-
-
December 13, 2023
OBSERVABLE UNIVERSE
- how far back? 4270
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