- 4619 - HUBBLE CONSTANT – of space expansion? We have been spoiled over recent years with first the Hubble Space Telescope (HST) and then the James Webb Space Telescope (JWST.) Both have opened our eyes on the Universe and made amazing discoveries. One subject that has received attention from both is the derivation of the Hubble Constant, a constant relating the velocity of remote galaxies and their distances.
--------------------------------------- 4619 - HUBBLE CONSTANT – of space expansion?
- The Hubble Constant (H0) is a fundamental
parameter in cosmology that defines the rate of expansion of the universe. It
defines the relationship between Earth and distant galaxies by the velocity
they are receding from us.
-
- It was first discussed by Edwin Hubble in
1929 as he observed the spectra of distant galaxies. It is measured in unites
of kilometres per second per megaparsec and shows how fast galaxies are moving
away from us per unit of distance. The exact value of the constant has been the
cause of many a scientific debate and more recently the HST and JWST have been
trying to fine tune its value. Getting an accurate value is key to determining
the age, size and fate of the universe.
-
- Astronomers used JWST to explore its earlier
results of the cepheid/supernova distance ladder. This has been used to
establish distances across the cosmos using cepheid variable stars and Type 1a
supernovae. Both objects can be likened to ‘standard candles’ whose “actual
brightness” is very well understood. By measuring their “apparent brightness”
from Earth, their distances can be calculated by comparing it to their actual
brightness, their 'intrinsic luminosity”.
-
- Over recent decades, a number of attempts
have been made to accurately determine H0 using a multitude of different
instruments and observations. The cosmic microwave background has been used
along with the studies using cepheid variables and supernovae events. The
results provide a range of results which has become known as ‘Hubble
tension.’
-
- To be able to determine H0 with a level of
accuracy using the cepheid/supernova ladder, a sufficiently high sample of
cepheids and supernovae must be observed. This has been challenging, in
particular of the sample size of supernovae within the range of cepheid
variable stars.
-
- They also explored other techniques for
determining H0 for example studying data from HST of the study of the
luminosity of the brightest red giant branch stars in a galaxy, which can also
work as a standard candle. Or the luminosity of certain carbon rich stars which
are another technique.
-
- Astronomers used three steps astronomers
used to measure the universe’s expansion rate (Hubble constant) to an
unprecedented accuracy, reducing the total uncertainty to 2.3 percent. The
“cosmic distance ladder” is used to measure accurate distances to galaxies near
to and far from Earth. The latest Hubble study extends the number of Cepheid
variable stars analyzed to distances of up to 10 times farther across our
galaxy than previous Hubble results.
-
- When all JWST measurements are combined,
including a correction for the low sample of supernovae data, that H0 comes out
at 72.6 ± 2.0 km s?1 Mpc? This compares
to the combined HST data which determines H0 as 72.8 km s?1 Mpc? It will take more years and more studies for
the sample size of supernova from JWST to equal that from HST but the
cross-check has so far revealed we are finally honing in on an accurate value
for Hubble’s Constant.
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November 23, 2024 HUBBLE CONSTANT – of space expansion? 4619
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--------------------- --- Sunday, November 24,
2024
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