- 4588 - UNIVERSE - You live in a golden age for learning about the universe because 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.
------------------------------------ 4588 - UNIVERSE - You live in a golden age?
- The dominant theoretical approach combines
“string theory”, a powerful mathematical framework with no successful physical
predictions as yet, and "cosmic inflation” 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.
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- The nature of the expected complexity could
take a bewildering variety of forms. On this basis, and 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 will 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. This possibility should be taken
far more seriously. For, if it 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.
-
- 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 tiny and curled
up.
-
- Unfortunately, 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.
-
- 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.
-
- “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.
-
- 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.
-
- The early extreme burst of expansion smoothed and flattened the universe and
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.
-
- Furthermore, 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.
-
- This 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.
-
- 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." 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.
-
- By exploiting this symmetry, one can follow
light and particles all the way back to the beginning. Doing so, we could
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. For example, 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).
-
- 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.
-
- Our 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.
-
- Likewise, our universe back through the big
bang is 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).
-
- Quantum theory, which rules the microcosmos
of atoms and particles, challenges our intuition, so at this point, the best we
can do is think of the mirror universe as a mathematical device which ensures
that the initial condition for the universe does not violate CPT symmetry.
-
- 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.
-
- 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.
-
- This mirror hypothesis allowed the
calculation to 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.
-
- Encouraged by this result led to another
big puzzle: why is the universe so uniform and spatially flat, not curved, on
the largest visible scales? “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. These kinds of arguments are used in
“statistical physics”, the field which underlies our understanding of heat,
work and thermodynamics.
-
- The late physicist Stephen Hawking famously
generalized statistical physics to include gravity. Using this elegant
argument, he calculated the temperature and the entropy of black holes. Using
this "mirror" hypothesis, astronomers managed to extend their
arguments to cosmology and to calculate the entropy of entire universe.
-
- To their surprise, 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.
-
- How would the “primordial density
variations”, usually attributed to inflation, have been generated in our
symmetrical mirror universe? Recently, they showed that 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.
-
- Even if this 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.
-
- By facing up to cosmology's deep puzzles,
guided by observations and exploring directions as yet unexplored, we may be
able to lay more secure foundations for both fundamental physics and our
understanding of the universe.
-
-
October 27, 2024 UNIVERSE - You
live in a golden age? 4588
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--------------------- --- Sunday, October 27,
2024
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