- 4072 - UNIVERSE - new theories for space-time? The Cosmological models we are using today are built on a simple, century-old idea, but new observations demand a radical rethink. Our ideas about the universe are based on a century-old simplification known as the cosmological principle. It suggests that when averaged on large scales, the Cosmos is homogeneous and matter is distributed evenly throughout.
------------ 4072 - UNIVERSE - new theories for space-time?
- The Cosmological
models we are using today are built on a simple, century-old idea, but new
observations demand a radical rethink.
Our ideas about the universe are based on a century-old simplification
known as the cosmological principle. It suggests that when averaged on large
scales, the Cosmos is homogeneous and matter is distributed evenly throughout.
-
- A homogeneus Cosmos
allows a mathematical description of space-time that simplifies the application
of Einstein's general theory of relativity to the universe as a whole. Our cosmological models are based on this
assumption.
-
- As new telescopes,
both on Earth and in space, deliver ever more precise images, and astronomers
discover massive objects such as the giant arc of quasars, this foundation from
general relstivity is increasingly challenged.
New discoveries force us to radically re-examine our assumptions and
change our understanding of the universe.
-
- Albert Einstein
faced huge dilemmas 106 years ago when he first applied his equations for
gravity to the universe as a whole. No physicist had ever attempted something
so bold, but it was a natural consequence of his key idea. “Matter tells space how to curve, and space
tells matter how to move."
-
- Data were almost completely
lacking in 1917 and the idea that galaxies were objects at vast distances was a
minority view among astronomers. The
conventional viewpoint, accepted by Einstein, was that the whole universe
looked like the inside of our galaxy. This suggested stars should be treated as
pressure-less fluids, distributed randomly but with a well defined average
density, the same, or homogeneous, anywhere in space.
-
- Based on that idea
that the universe is the same everywhere, Einstein introduced his cosmological
constant Λ, now known as "dark energy." On small scales, Einstein's equations tell us
that space never stands still. But forcing this on the universe on a large
scale was unnatural. Einstein was therefore relieved by the discovery of the
expanding universe in the late 1920s. He even described this as his biggest
blunder.
-
- Ideas about
matter have evolved, but not geometry.
We now have amazingly detailed models of the physics of stars and
galaxies embedded in the evolving universe. We can trace the astrophysics of
"stuff" from tiny seed ripples in the primordial fireball all the way
to complex structures today.
-
- Our telescopes are
wonderful time machines. They look back all the way to when the first atoms
formed, and the universe first became transparent. Beyond is the “primordial plasma”, opaque
like the interior and surface of the sun. The light that left the universe's
"surface of last scattering" was very hot back then, about 2,700℃.
-
- We receive that
same light today, but cooled to minus 270℃ and diluted by the expansion of the
universe. This is the “cosmic microwave background” and it is remarkably
uniform in all directions.
-
- This is strong
evidence the universe was very close to spatially uniform when it was a fireball.
But there is no direct evidence for such uniformity today. Far back in time, our telescopes reveal small
merging galaxies, growing into ever larger structures until the present day.
-
- The expansion of
the universe has been halted entirely within the largest matter concentrations
known as galaxy clusters. Where space is expanding, the clusters are stretched
in filaments and sheets that thread and surround vast empty voids, all growing
with time but at different rates. Rather than being smooth, matter forms a
"cosmic web".
-
- But the idea that
the universe is spatially homogeneous endures.
There would be a gross inconsistency between the observed cosmic web and
an average curved geometry of space if all we see is all there is. Evidence for
missing matter has been around since the first observations of galaxy clusters
in 1933.
-
- Our first
observations of the cosmic microwave background radiation and its ripples in
the decade from 1965 changed that idea.
Our models of nuclear physics are wonderfully predictive. But they are
only consistent with observations if the missing mass in galaxy clusters is
something like neutrinos that cannot emit light. Thus we invented “cold dark
matter”, which makes gravity stronger within galaxy clusters.
-
- Billions have been
spent trying to directly detect dark matter, but decades of such efforts have
yielded no definitive detection of what makes up 80% of all matter and 20% of
all the energy in the universe today.
-
- The cosmic
microwave background radiation is not perfectly uniform. Superimposed on it are
fluctuations, one of which is abnormally large and has the shape of a dipole
covering the whole sky.
-
- We can interpret
this as an effect due to relative motion, provided we define the cosmic microwave
background radiation as the rest frame of the universe. If we didn't do this,
we would need a physical explanation for this large dipole.
-
- Much of this puzzle
boils down to a power asymmetry, a lopsided universe. The temperatures of the
hemispheres above and below the plane of the Milky Way are slightly different
to expectation.
-
- These anomalies
have long been explained as a result of unaccounted physical processes in
modeling microwave emissions from the Milky Way. The cosmic microwave background radiation is
not the only all-sky observation to show a dipole.
-
- Last year,
2022, researchers used observations of
1.36 million distant quasars and 1.7 million radio sources to test the
cosmological principle. They found that matter, too, is unevenly distributed.
-
- Another even more
widely discussed mystery is the "Hubble tension." We assume that an all-sky average of the
universe's present expansion rate gives one well defined value: the “Hubble
constant”. But the measured value differs from expectation, given a standard
expansion history based on the cosmic microwave background radiation. If we
allowed for inhomogeneous cosmologies, this problem may simply disappear.
-
- Using cosmic
microwave background data from individual opposing hemispheres, a standard
expansion history implies different Hubble "constants" on each side
of the sky today. These puzzles are
compounded by an ever-growing list of unexpected discoveries: a vast giant arc
of quasars and a complex, bright and element-filled early universe unveiled by
the James Webb Space Telescope.
-
- If matter is much
more varied and interesting than expected, then maybe the geometry is too. Models which abandon the cosmological
principle do exist and make predictions. They are simply less studied than
“standard cosmology”.
-
- The European Space
Agency's Euclid satellite will be launched this year, 2023. Will Euclid reveal
that on average space is not Euclidean? If so, then a fundamental revolution in
physics might be around the corner.
-
- Our cosmological
models are based on this earlier assumptions. But as new telescopes, both on
Earth and in space, deliver ever more precise images, and astronomers discover
massive objects such as the giant arc of quasars, this foundation is
increasingly challenged.
- We now have
amazingly detailed models of the physics of stars and galaxies embedded in the
evolving universe. We can trace the astrophysics of "stuff" from tiny
seed ripples in the primordial fireball all the way to complex structures
today.
-
- Our telescopes are
wonderful time machines. They look back all the way to when the first atoms
formed, and the universe first became transparent. Beyond is the primordial plasma, opaque like
the interior and surface of the sun. The light that left the universe's
"surface of last scattering" was very hot back then, about 2,700℃.
-
- We receive that
same light today, but cooled to minus 270℃ and diluted by the expansion of the
universe. This is the cosmic microwave background and it is remarkably uniform
in all directions. This is strong
evidence the universe was very close to spatially uniform when it was a
fireball. But there is no direct evidence for such uniformity today.
-
- Far back in time,
our telescopes reveal small merging galaxies, growing into ever larger
structures until the present day. The
expansion of the universe has been halted entirely within the largest matter
concentrations known as galaxy clusters. Where space is expanding, the clusters
are stretched in filaments and sheets that thread and surround vast empty
voids, all growing with time but at different rates. Rather than being smooth,
matter forms a "cosmic web".
-
- There would be a
gross inconsistency between the observed cosmic web and an average curved
geometry of space if all we see is all there is. Evidence for missing matter
has been around since the first observations of galaxy clusters in 1933.
-
- Our first
observations of the cosmic microwave background radiation and its ripples in
the decade from 1965 changed that idea.
Our models of nuclear physics are wonderfully predictive. But they are
only consistent with observations if the missing mass in galaxy clusters is
something like neutrinos that cannot emit light. Thus we invented cold dark
matter, which makes gravity stronger within galaxy clusters.
-
- The cosmic
microwave background radiation is not the only all-sky observation to show a
dipole. Last year, researchers used observations of 1.36 million distant quasars
and 1.7 million radio sources to test the cosmological principle. They found
that matter, too, is unevenly distributed.
-
- Another even more
widely discussed mystery is the "Hubble tension." We assume that an all-sky average of the
universe's present expansion rate gives one well defined value: the Hubble
constant. But the measured value differs from expectation, given a standard
expansion history based on the cosmic microwave background radiation. If we
allowed for inhomogeneous cosmologies, this problem may simply disappear.
-
- Using cosmic
microwave background data from individual opposing hemispheres, a standard
expansion history implies different Hubble "constants" on each side
of the sky today. These puzzles are
compounded by an ever-growing list of unexpected discoveries: a vast giant arc
of quasars and a complex, bright and element-filled early universe unveiled by
the James Webb Space Telescope.
-
- If matter is much
more varied and interesting than expected, then maybe the geometry is too. Models which abandon the cosmological
principle do exist and make predictions. They are simply less studied than
standard cosmology.
-
- The European Space
Agency's Euclid satellite will be launched this year. Will Euclid reveal that
on average space is not Euclidean? If so, then a fundamental revolution in
physics might be around the corner.
-
June 30, 2023 UNIVERSE - new
theories for space-time?
4063
------------------------------------------------------------------------------------------
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