- 4156 - UNIVERSE EXPANSION RATE? The Hubble Constant is one of the fundamental parameters for understanding the evolution and ultimate fate of the universe. However, a persistent difference called the "Hubble Tension" is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the big bang afterglow.
---------------------- 4156 - UNIVERSE EXPANSION RATE?
- The rate at which the universe is
expanding, known as the “Hubble constant”, ”H”. The constant rate of expansion
is about 49,300 miles per hour for every million light year distance. The greater the distance the faster the
expansion rate.
-
- The James Webb Space Telescope provides new
capabilities to scrutinize and refine some of the strongest observational
evidence for this tension. The sign
cosmologists want to read is a cosmic speed limit sign that tells us how fast
the universe is expanding. We call that
number called the “Hubble constant”.
-
- The brightness of certain stars in those
galaxies tell us how far away they are and thus for how much time this light
has been traveling to reach us, and the redshifts of the galaxies tell us how
much the universe expanded over that time, hence telling us the expansion rate.
-
- Using the combined power of the NASA’s
Hubble and Webb space telescopes astronomers are nailing down precise distances
to a special class of variable star that is used in calibrating the expansion
rate of the universe. These “Cepheid variable stars” are seen in crowded star
fields.
-
- Light contamination from surrounding stars
may make the measurement of the brightness of a Cepheid less precise. Webb’s
sharper infrared vision allows for a Cepheid target to be more clearly isolated
from surrounding stars. The Webb data
confirms the accuracy of 30 years of Hubble observations of Cepheids that were
critical in establishing the bottom rung of the cosmic distance ladder for
measuring the universe’s expansion rate.
-
- This particular class of stars,” Cepheid
variables”, has given us the most precise measurements of distance for over a
century because these stars are extraordinarily bright. They are supergiant stars, a hundred thousand
times the luminosity of the sun. They pulsate, expand and contract in size,
over a period of weeks that indicates their relative luminosity. The longer the
period, the intrinsically brighter they are.
-
- They are the gold standard tool for the
purpose of measuring the distances of galaxies a hundred million or more light
years away, a crucial step to determine the Hubble constant.
-
- Prior to Hubble's 1990 launch and its
subsequent Cepheid measurements, the expansion rate of the universe was so
uncertain astronomers weren't sure if the universe has been expanding for 10
billion or 20 billion years. That's because a faster expansion rate will lead
to a younger age for the universe, and a slower expansion rate will lead to an
older age of the universe.
-
- Hubble has better visible-wavelength
resolution than any ground-based telescope because it sits above the blurring
effects of Earth's atmosphere. As a result, it can identify individual Cepheid
variables in galaxies that are more than a hundred million light-years away and
measure the time interval over which they change their brightness.
-
- However, we also must observe the Cepheids
at the near-infrared part of the spectrum to see the light which passes
unscathed through intervening dust. Dust
absorbs and scatters blue optical light, making distant objects look faint and
fooling us into believing they are farther away than they are.
-
- Hubble's red-light vision is not as sharp
as its blue, so the Cepheid starlight we see there is blended with other stars
in its field of view. We can account for the average amount of blending,
statistically, the same way a doctor figures out your weight by subtracting the
average weight of clothes from the scale reading, but doing so adds noise to
the measurements. Some people's clothes are heavier than others.
-
- Sharp infrared vision is one of the James
Webb Space Telescope's superpowers. With its large mirror and sensitive optics,
it can readily separate the Cepheid light from neighboring stars with little
blending.
-
- The first step along the distance
ladder involves observing Cepheids in a
galaxy with a known, geometric distance that allows us to calibrate the true
luminosity of Cepheids.
-
- The second step is to observe Cepheids in
the host galaxies of recent Type Ia supernovae.
The combination of the first two steps transfers knowledge of the
distance to the supernovae to calibrate their true luminosities.
-
- Step three is to observe those supernovae
far away where the expansion of the universe is apparent and can be measured by
comparing the distances inferred from their brightness and the redshifts of the
supernova host galaxies. This sequence of steps is known as the “distance
ladder."
-
- Hubble Webb's measurements have
dramatically cut the noise in the Cepheid measurements due to the observatory's
resolution at near-infrared wavelengths.
Astronomers observed more than 320 Cepheids across the first two steps.
They confirmed that the earlier Hubble Space Telescope measurements were
accurate, but noisier.
-
- What the results still do not explain is why
the universe appears to be expanding so fast! We can predict the expansion rate
of the universe by observing its baby picture, the cosmic microwave background,
and then employing our best model of how it grows up over time to tell us how
fast the universe should be expanding today.
-
- The fact that the present measure of the
expansion rate significantly exceeds the prediction is a now decade-long
problem called 'The Hubble Tension'. The most exciting possibility is that the
Tension is a clue about something we are missing in our understanding of the
universe.
-
- It may indicate the presence of exotic dark
energy, exotic dark matter, a revision to our understanding of gravity, or the
presence of a unique particle or field. The more mundane explanation would be
multiple measurement errors conspiring in the same direction (astronomers have
ruled out a single error by using independent steps), so that is why it is so
important to redo the measurements with greater fidelity.
-
- With Webb confirming the measurements from
Hubble, the Webb measurements provide the strongest evidence yet that
systematic errors in Hubble's Cepheid photometry do not play a significant role
in the present Hubble Tension. As a result, the more interesting possibilities
remain on the table and the mystery of the Tension deepens.
-
-
September 16, 2023 UNIVERSE EXPANSION
RATE? 4156
------------------------------------------------------------------------------------------
-------- Comments
appreciated and Pass it on to whomever is interested. ---
--- Some reviews are
at: -------------- http://jdetrick.blogspot.com -----
-- email feedback,
corrections, request for copies or Index of all reviews
--- to: ------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- ---
Sunday, September 17, 2023 ---------------------------------
No comments:
Post a Comment