- 4166 - HUBBLE EXPANSON RATE ? According to most models, the Hubble constant should equal something around 68 kilometers per second per megaparsec (km/s/Mpc). One megaparsec is 1,000,000 parsecs, or about 3,260,000 light-years. But after scanning stars and galaxies across our universe, some experts calculate the constant to be 69.8 km/s/Mpc, while others find it to be as high as 74 km/s/Mpc, depending on the method of measurement.
-------------------------------- 4166 - HUBBLE EXPANSON RATE ?
- One of the biggest
and most heated cosmic debates of our time surrounds a peculiar dilemma the
“Hubble tension”. This phrase describes
the fact that, even though scientists are aware the cosmos is constantly
ballooning outward in every direction.
We can clearly see stars and galaxies drifting farther and farther away
from us over time. The rate is
accelerating, a startling discovery astronomers made in the late 1990s that
could be due in part to the existence of dark energy.
-
- The James Webb
Space Telescope (JWST) has weighed in on the situation for the first time, but
it did not solve the mystery. In fact, JWST actually thickened it.
-
- The problem with
calculating the rate is that it depends on resolving the true value of the
Hubble constant, which is a crucial number in calculating the universe's
expansion rate. Yet, for whatever reason, our theoretical predictions of the
constant do not appear to match up with reality.
-
- According to most
models, the Hubble constant should equal something around 68 kilometers per
second per megaparsec (km/s/Mpc). One megaparsec is 1,000,000 parsecs, or about
3,260,000 light-years. But after scanning stars and galaxies across our
universe, some experts calculate the constant to be 69.8 km/s/Mpc, while others
find it to be as high as 74 km/s/Mpc, depending on the method of measurement.
-
- Perhaps the models
that presently thread our understanding of the universe are missing
something? The JWST's results have
crossed one more item off that list. In a nutshell, it showed that the
so-called crisis is probably not due to technical issues with measurements made
by the Hubble Space Telescope. Back in the 1920s, the American astronomer Edwin
Hubble discovered that the universe is expanding.
-
- The most common
features that scientists use to decode the Hubble constant are from Cepheid
stars. 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.
-
- Comparison of
Cepheid period-luminosity relations are used to measure distances. Hubble is a
key device used in resolving Hubble tension because it's able to measure
stellar brightnesses with incredible precision.
It sits above Earth's blurring atmosphere, unlike ground-based
observatories hampered by our planet's hazy shield.
-
- Such brightnesses
can tell us how far away those stars are and, because we know the immutable
speed of light, for how long that light has been traveling to reach us. After
some calculations, scientists reason that this kind of information taken from
lots and lots of stars should help us figure out the Hubble constant.
-
- Prior to Hubble’s
1990 launch 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. There is one star in particular
that scientists like to focus on with Hubble to tease out the universe's
expansion rate: Cepheids. These are supergiant stars with something like
100,000 times the luminosity of our sun.
-
- Cepheids pulsate,
expand and contract in size, which indicates their relative luminosities. The
longer the period the intrinsically brighter they are, and this provides baseline brightnesses and
ultimately more accurate measurements.
-
- The Hubble telescope perched above our atmosphere
can identify individual Cepheids in galaxies more than a hundred million
light-years away, thus measuring the time interval over which these galaxies
change their brightness.
-
- The combined power
of the Hubble and Webb space telescopes nails 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.
-
- It's not quite
sensitive enough to infrared light wavelengths, which are found beyond the red
end of the electromagnetic spectrum and remain invisible to human eyes.
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.
-
- Infrared vision is
important when peering at faraway objects because light coming from distant
sources gets stretched out on the way to our vantage point on Earth. Once-tight
bluish wavelengths turn into longer, reddish ones. That's actually where the
term "redshifted galaxies" comes from, referring to realms falling
deeper toward that end of the spectrum from our ground-based perspective.
-
- Only infrared light
has the ability to pass through dust unscathed, meaning if a Cepheid is stuck
behind a shroud of interstellar matter, it'd appear fainter to us. That runs
the risk of its light blending in with light from another Cepheid in the
vicinity, for instance, or making it seem like a star is farther away than it
truly is.
-
- The James Webb
Space Telescope is a $10 billion observatory, sitting nearly 1 million miles
away from Earth, is built to unveil the infrared universe to us.
-
- The first step
involved observing Cepheids in a galaxy with a known geometric distance for
calibration purposes. That galaxy was NGC 4258. The second step was to observe
Cepheids in the host galaxies of recent Type Ia supernovas, which are bright
star explosions, to basically double check whether Hubble's observations were
right.
-
-
September 25, 2023 HUBBLE EXPANSON
RATE ? 4166
------------------------------------------------------------------------------------------
-------- 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” -----------
--------------------- ---
Tuesday, September 26, 2023 ---------------------------------
No comments:
Post a Comment