- 4587 -
UNIVERSE - how big is it? We live in a golden age for learning about
the universe. Our most powerful telescopes have revealed that the universe is
surprisingly simple on the largest visible scales. Likewise, our most powerful
"microscope," the Large Hadron Collider, has found no deviations from
known physics on the tiniest scales.
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-------------------------------------------- 4587
- UNIVERSE - how
big is it?
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- The dominant theoretical ideas combines
string theory, a powerful mathematical framework with no successful physical
predictions, and "cosmic inflation", the idea that, at a very early
stage, the universe ballooned wildly in size. In combination, string theory and
inflation predict the universe to be incredibly complex on tiny scales and
completely chaotic on very large scales.
What gives?
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- The nature of the expected complexity could
take a bewildering variety of forms.
Despite the absence of observational evidence, many theorists promote
the idea of a "multiverse": an uncontrolled and unpredictable cosmos
consisting of many universes, each with totally different physical properties
and laws.
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- So far, the observations indicate exactly
the opposite. What should we make of the discrepancy? One possibility is that
the apparent simplicity of the universe is merely an accident of the limited
range of scales we can probe today, and that when observations and experiments
reach small enough or large enough scales, the complexity may be revealed.
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- The other possibility is that the universe
really is very simple and predictable on both the largest and smallest scales.
If this is true, we may be closer than we imagined to understanding the
universe's most basic puzzles. And some of the answers may already be staring
us in the face.
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- The current orthodoxy is the culmination of
decades of effort by thousands of serious theorists. According to string
theory, the basic building blocks of the universe are miniscule, vibrating
loops and pieces of sub-atomic string. As currently understood, the theory only
works if there are more dimensions of space than the three we experience. So,
string theorists assume that the reason we don't detect them is that they are
too tiny and curled up.
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- This makes string theory hard to test,
since there are an almost unimaginable number of ways in which the small
dimensions can be curled up, with each giving a different set of physical laws
in the remaining, large dimensions.
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- Meanwhile, “cosmic inflation” is a scenario
proposed in the 1980s to explain why the universe is so smooth and flat on the
largest scales we can see. The idea is that the infant universe was small and
lumpy, but an extreme burst of ultra-rapid expansion blew it up vastly in size,
smoothing it out and flattening it to be consistent with what we see today.
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- “Inflation” is also popular because it
potentially explains why the energy density in the early universe varied
slightly from place to place. This is important because the denser regions would
have later collapsed under their own gravity, seeding the formation of
galaxies.
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- Over the past three decades, the density
variations have been measured more and more accurately both by mapping the
cosmic microwave background, the radiation from the big bang, and by mapping
the three-dimensional distribution of galaxies.
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- In most models of inflation, the early
extreme burst of expansion which smoothed and flattened the universe also
generated long-wavelength gravitational waves, ripples in the fabric of
space-time. Such waves would be a "smoking gun" signal confirming
that inflation actually took place. However, so far the observations have
failed to detect any such signal. Instead, as the experiments have steadily
improved, more and more models of inflation have been ruled out.
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- During inflation, different regions of space
can experience very different amounts of expansion. On very large scales, this
produces a multiverse of post-inflationary universes, each with different
physical properties.
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- The “inflation scenario” is based on
assumptions about the forms of energy present and the initial conditions. While
these assumptions solve some puzzles, they create others. String and inflation
theorists hope that somewhere in the vast inflationary multiverse, a region of
space and time exists with just the right properties to match the universe we
see.
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- Over the past several decades, there have
been many opportunities for experiments and observations to reveal specific
signals of string theory or inflation. But none have been seen. Again and
again, the observations turned out simpler and more minimal than anticipated.
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- One of cosmology's greatest paradoxes, if
we follow the expanding universe backward in time, using Einstein's theory of
gravity and the known laws of physics, space shrinks away to a single point,
the "initial singularity."
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- In trying to make sense of this infinitely
dense, hot beginning, theorists pointed to a deep symmetry in the basic laws
governing light and massless particles. This symmetry, called
"conformal" symmetry, means that neither light nor massless particles
actually experience the shrinking away of space at the big bang.
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- Astronomers can describe the initial
“singularity” as a "mirror", a reflecting boundary in time (with time
moving forward on one side, and backward on the other). Picturing the big bang as a mirror neatly
explains many features of the universe which might otherwise appear to conflict
with the most basic laws of physics.
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- For every physical process, quantum theory
allows a "mirror" process in which space is inverted, time is
reversed and every particle is replaced with its anti-particle (a particle
similar to it in almost all respects, but with the opposite electric charge).
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- According to this powerful symmetry, called
“CPT symmetry”, the "mirror" process should occur at precisely the
same rate as the original one. One of the most basic puzzles about the universe
is that it appears to violate CPT symmetry because time always runs forward and
there are more particles than anti-particles.
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- The
“mirror hypothesis” restores the symmetry of the universe. When you look
in a mirror, you see your mirror image behind it: if you are left-handed, the
image is right-handed and vice versa. The combination of you and your mirror
image is more symmetrical than if you are alone.
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- Likewise, extrapolating our universe back
through the big bang, its mirror image, a pre-bang universe in which (relative
to us) time runs backward and antiparticles outnumber particles. For this
picture to be true, we don't need the mirror universe to be real in the
classical sense (just as your image in a mirror isn't real).
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- We think of the mirror universe as a
mathematical device which ensures that the initial condition for the universe
does not violate CPT symmetry.
Surprisingly, this new picture provided an important clue to the nature
of the unknown cosmic substance called “dark matter”. -
- Neutrinos are very light, ghostly particles
which, typically, move at close to the speed of light and which spin as they
move along, like tiny tops. If you
point the thumb of your left hand in the direction the neutrino moves, then
your four fingers indicate the direction in which it spins. The observed, light
neutrinos are called "left-handed" neutrinos.
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- Heavy "right-handed" neutrinos
have never been seen directly, but their existence has been inferred from the
observed properties of light, left-handed neutrinos. Stable, right-handed
neutrinos would be the perfect candidates for dark matter because they don't
couple to any of the known forces except gravity.
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- This mirror hypothesis allowed us to
calculate exactly how many would form, and to show they could explain the
cosmic dark matter. If the dark matter
consists of stable, right-handed neutrinos, then one of three light neutrinos
that we know of must be exactly massless.
This prediction is now being tested using observations of the
gravitational clustering of matter made by large-scale galaxy surveys.
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- Why is the universe so uniform and
spatially flat, not curved, on the largest visible scales? The cosmic inflation
scenario was invented by theorists to solve this problem.
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- Entropy is a concept which quantifies the
number of different ways a physical system can be arranged. For example, if we
put some air molecules in a box, the most likely configurations are those which
maximize the entropy, with the molecules more or less smoothly spread
throughout space and sharing the total energy more or less equally.
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- Physicists calculated the temperature and
the entropy of black holes using this "mirror" hypothesis. The universe with the highest entropy
(meaning it is the most likely, just like the atoms spread out in the box) is
flat and expands at an accelerated rate, just like the real one. So statistical
arguments explain why the universe is “flat and smooth” and has a small
positive accelerated expansion, with no need for cosmic inflation.
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- How would the primordial density variations,
usually attributed to inflation, have been generated in our symmetrical mirror
universe? A specific type of quantum
field (a dimension zero field) generates exactly the type of density variations
we observe, without inflation. These density variations aren't accompanied by
the long wavelength gravitational waves which inflation predicts, and which
haven't been seen.
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- These results are very encouraging. But more
work is needed to show that our new theory is both mathematically sound and
physically realistic. Even if our new
theory fails, it has taught us a valuable lesson. There may well be simpler,
more powerful and more testable explanations for the basic properties of the
universe than those the standard orthodoxy provides.
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October 26, 2024 UNIVERSE - how
big is it? 4587
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--------------------- --- Sunday, October 27,
2024
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