- 4148 - UNIVERSE - model of the beginning? Our understanding of the Universe is rooted in a cosmological model known as LCDM. The CDM stands for Cold Dark Matter, where most of the matter in the universe isn’t stars and planets, but a strange form of matter that is dark and nearly invisible.
-------------- 4148 - UNIVERSE - model of the beginning?
- The “L”, or Lambda, represents dark energy.
It is the symbol used in the equations of general relativity to describe the
“Hubble parameter”, or the “rate of cosmic expansion”. Although the LCDM model
matches our observations incredibly well, it isn’t perfect. And the more data
we gather on the early Universe, the less perfect it seems to be.
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- A central difficulty is the fact that
increasingly our various measures of the Hubble parameter aren’t lining
up. If we use fluctuations in the cosmic
microwave background to calculate the parameter, we get a value of about 68
km/s per megaparsec. If we look at distant supernova to measure it, we get a
value of around 73 km/s per megaparsec.
-
- In the past, the uncertainty of these
values was large enough that they overlapped, but we’ve now measured them with
such precision that they truly disagree. This is known as the “Hubble Tension
problem”, and it’s one of the deepest mysteries of cosmology at the moment.
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- Much of the effort to solve this mystery
has focused on better understanding the nature of dark energy. In Einstein’s
early model, cosmic expansion is an inherent part of the structure of space and
time. A cosmological constant that expands the Universe at a steady rate.
-
- But perhaps dark energy is an exotic
“scalar field”, one that would allow a variable expansion rate or even an
expansion that varies slightly depending on which direction you look. Maybe the
rate was greater in the period of early galaxies, then slowed down, hence the
different observations. We know so little about dark energy that there are lots
of theoretical possibilities.
-
- If you know the age of a star or galaxy a
billion light-years away, then you know the Universe must have been at least
that old a billion years ago. If this age disagrees with LCDM, then LCDM must
be wrong.
-
- There are a few stars that appear to be older
than the Universe, which big bang skeptics often point to as disproving the big
bang. This doesn’t work because the age of these stars is uncertain enough to
be younger than the Universe. But you can expand upon the idea as a
cosmological test. Determine the age of thousands of stars at various
distances, then use statistics to gauge a minimum cosmological age at different
epochs, and from that calculate a minimum Hubble parameter.
-
- Determining the age of stars and globular
clusters is particularly difficult, so the resulting data is a bit fuzzy. While
it’s possible to fit the data to the range of Hubble parameters we have from
direct measures, the age-distance data suggests the Universe is a bit older
than the LCDM allows. If the age data is
truly accurate, there is a discrepancy between cosmic age and stellar ages.
-
- Regular matter is made of baryons and
leptons. The protons and neutrons in an atom are 'baryons', and the electrons
are 'leptons'. So Baryonic matter is the usual type of matter we see every day,
as opposed to dark matter.
-
- Baryon Acoustic Oscillation (BAO) refers to
the fluctuations of matter density in the early Universe. Back when the
Universe was in a hot dense state, these fluctuations rippled through the
cosmos like sound waves. As the Universe expanded, the more dense regions
formed the seeds for galaxies and galactic clusters. The scale of those
clusters is driven by cosmic expansion. So by looking at BAO across the
Universe, we can study the evolution of dark energy over time.
-
- BAO and CMB agree, but barely. What’s nice about BAO is that it connects
the distribution of galaxies we see today to the inflationary state of the
Universe during the period of the cosmic microwave background (CMB). It’s a way
to compare the value of the early Hubble parameter with the more recent value.
-
- This is because early inflation put a limit
on how far acoustic waves could propagate. The higher the rate of expansion
back then, the smaller the acoustic range. It’s known as the 'acoustic
horizon', and it depends not only on the expansion rate but also on the density
of matter at the time.
-
- When we compare BAO and CMB observations,
they do agree, but only for a level of matter on the edge of observed limits.
In other words, if we get a better measure of the density of matter in the
Universe, we could have a CMB/BAO tension just as we currently have a Hubble
Tension.
-
- Both the supernovae and cosmic microwave
background measures of the Hubble parameter depend on a scaffold of
interlocking models. The supernova measure depends on the cosmic distance
ladder, where we use various observational models to determine ever greater
distances. The CMB measure depends on the LCDM model, which has some
uncertainty in its parameters such as matter density. Cosmic chronometers are
observational measures of the Hubble parameter that aren’t model dependent.
-
- One of these measures uses “astrophysical
masers”. Under certain conditions, hot matter in the accretion disk of a black
hole can emit microwave laser light. Since this light has a very specific
wavelength, any shift in that wavelength is due to the relative motion or
cosmic expansion, so we can measure the expansion rate directly from the
overall redshift of the “maser', and we can measure the distance from the scale
of the accretion disk. Neither of these require cosmological model assumptions.
-
- Another approach uses “gravitational
lensing”. If a nearby galaxy happens to be between us and a distant supernova,
the light from the supernova can be gravitationally lensed around the galaxy,
creating multiple images of the supernova. Since the light of each image
travels a different path, each image takes a different amount of time to reach
us. When we are lucky we can see the supernova multiple times. By combining
these observations we can get a direct measure of the Hubble parameter, again
without any model assumptions.
-
- The maser method gives a Hubble parameter of
about 72 – 77 (km/s)/Mpc, while the gravitational lensing approach gives a
value of about 63 – 70 (km/s)/Mpc. These results are tentative and fuzzy, but
it looks as if even model-independent measures of the Hubble parameter won’t
eliminate the tension problem.
-
- Within general relativity the Hubble
parameter is constant. The Lambda is a cosmological constant, driving expansion
at a steady pace. This means that the density of dark energy is uniform
throughout time and space.
-
- Some exotic unknown energy might drive
additional expansion, but in the simplest model, it should be constant. So the
redshifts of distant galaxies should be directly proportional to distance.
There may be some small variation in redshift due to the actual motion of
galaxies through space, but overall there should be a simple redshift relation.
-
- But there’s some evidence that the Hubble
parameter isn’t constant. A survey of distant quasars gravitationally lensed by
closer galaxies calculated the Hubble value at six different redshift
distances. The uncertainties of these values are fairly large, but the results
don’t seem to cluster around a single value. Instead, the Hubble parameter for
closer lensings seems higher than those of more distant lensings. The best fit
puts the Hubble parameter at about 73 (km/s)/Mpc, but that assumes a constant
value.
-
- When we look at light from the cosmic
microwave background The CMB light has to travel across billions of light-years
to reach us, and that means it often has to pass through dense regions of
galaxy clusters and the vast voids between galaxies. As it does so, the light
can be red-shifted or blue-shifted by the gravitational variations of the
clusters and voids. As a result, regions of the CMB can appear warmer or cooler
than it actually is. This is known as the “Integrated Sachs-Wolfe (ISW)
effect”.
-
- When we look at fluctuations within the
CMB, most of them are on a scale predicted by the LCDM model, but there are
some larger scale fluctuations that are not, which we call anomalies. Most of
these anomalies can be accounted for by the Integrated Sachs-Wolfe effect.
-
- In general, our cosmological model depends
on two parameters: the fraction of dark energy and the fraction of matter. Just
as dark energy drives cosmic expansion, working to move galaxies away from each
other, dark matter and regular matter work against cosmic expansion. We mostly
see the effect of matter density through the clustering of galaxies, but the
overall density of matter in the Universe also dampens the observed expansion
rate.
-
- The “cosmic matter density” can be
determined by many of the same observational tests used to determine cosmic
expansion. All of them are in general agreement that the matter density is
about 30% of the total mass-energy of the Universe, but the early Universe
observations trend a bit lower.
Increasing the expansion rate of the early universe would tend to make
this problem worse, not better.
-
- “Power spectrum” doesn’t have to do with the
amount of energy a galaxy has, but rather the scale at which galaxies cluster.
If you look at the distribution of galaxies across the entire Universe, you see
small galaxy clusters, big galaxy clusters, and everything in between. At some
scales clusters are more common and at others more rare. So one useful tool for
astronomers is to create a “power spectrum” plotting the number of clusters at
each scale.
-
- The “galaxy power spectrum” depends upon
both the matter and energy of the Universe. It’s also affected by the initial
hot dense state of the Big Bang, which we can see through the cosmic microwave
background. Several galactic surveys have measured the galactic power spectrum,
such as the “Baryon Oscillation Spectroscopic Survey” (BOSS). Generally, they
point to a lower rate of cosmic expansion closer to those of the cosmic
microwave background results.
-
- One thing that should be emphasized is that
none of these results in any way disprove the big bang. On the whole, our
standard model of cosmology is on very solid ground. What it does show is that
the Hubble Tension problem isn’t the only one hovering at the edge of our
understanding.
-
- Independently we know from DES Y3 papers
that LCDM seems robust, specifically the cosmological equation of state agrees
with all the observations thus far. A parallel would be the history of
measurements of the speed of light in vacuum, which spread some in the
beginning but had started to converge on a value within the estimated
measurement precision.
-
-
September 10,
2023 UNIVERSE - model of the beginning? 4148
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