- 4387
- WHAT IS THE HUBBLE CONSTANT? The rate at which the universe is expanding
is the “Hubble constant”. It is one of
the fundamental parameters for understanding the evolution and ultimate fate of
the universe.
-
4387 - WHAT IS THE HUBBLE CONSTANT?
- 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
afterglow of the Big Bang.
-
- One of the scientific justifications for
building the NASA/ESA Hubble Space Telescope was to use its observing power to
provide an exact value for the expansion rate of the universe. Prior to
Hubble's launch in 1990, observations from ground-based telescopes yielded huge
uncertainties. Depending on the values deduced for the expansion rate, the
universe could be anywhere between 10 and 20 billion years old.
-
- Over the past 34 years, Hubble has shrunk
this measurement to an accuracy of less than one percent, splitting the
difference with an age value of 13.8 billion years. This has been accomplished
by refining the so-called 'cosmic distance ladder' by measuring important
milepost markers known as Cepheid variable stars.
-
- However, the Hubble value does not agree
with other measurements that imply that the universe was expanding faster after
the Big Bang. These observations were made by the ESA Planck satellite's
mapping of the cosmic microwave background radiation.
-
- The James Webb Space Telescope enabled
astronomers to crosscheck Hubble's results. Webb's infrared views of Cepheids
agreed with Hubble's optical-light data. Webb confirmed that the Hubble
telescope's keen eye was right all along, erasing any lingering doubt about
Hubble's measurements.
-
- The so-called 'Hubble Tension ” between what happens in the
nearby universe compared to the early universe's expansion remains a nagging
puzzle for cosmologists. There may be something woven into the fabric of space
that we don't yet understand.
-
- Does resolving this discrepancy require new
physics? Or is it a result of measurement errors between the two different
methods used to determine the rate of expansion of space? Hubble and Webb have now tag-teamed to
produce definitive measurements, furthering the case that something else ”not
measurement errors”is influencing the expansion rate.
-
- With measurement errors negated, what
remains is the real and exciting possibility that we have misunderstood the
universe. The fact that the universe's
expansion is accelerating, owing to a mysterious phenomenon now called “dark
energy”.
-
- Stellar crowding could affect brightness
measurements of more distant stars in a systematic way. Astronomers use various methods to measure
relative distances in the universe, depending on the object being observed.
Collectively these techniques are known as the “cosmic distance ladder” each
rung or measurement technique relies upon the previous step for calibration.
-
- But some astronomers suggested that moving
outward along the 'second rung,' the cosmic distance ladder might get shaky if
the Cepheid measurements become less accurate with distance. Such inaccuracies
could occur because the light of a Cepheid could blend with that of an adjacent
star, an effect that could become more pronounced with distance as stars crowd
together in the sky and become harder to distinguish from one another.
-
- The observational challenge is that past
Hubble images of these more distant Cepheid variables look more huddled and
overlapping with neighboring stars at ever greater distances between us and
their host galaxies, requiring careful accounting for this effect.
-
- Intervening dust further complicates the
certainty of the measurements in visible light. Webb slices through the dust
and naturally isolates the Cepheids from neighboring stars because its vision
is sharper than Hubble's at infrared wavelengths.
-
- The new Webb observations include five host
galaxies of eight Type Ia supernovae containing a total of 1000 Cepheids and
reach out to the farthest galaxy where Cepheids have been well measured ”NGC
5468, at a distance of 130 million light-years.
-
- This spans the full range where we made
measurements with Hubble. So, we've gone to the end of the second rung of the
cosmic distance ladder.
-
- Together, Hubble's and Webb's confirmation
of the Hubble Tension sets up other observatories to possibly settle the
mystery, including NASA's upcoming Nancy Grace Roman Space Telescope and ESA's
recently launched Euclid mission.
-
-
March 13, 2024 WHAT IS THE HUBBLE CONSTANT? 4387
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Pass it on to whomever is interested. --------
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------ “Jim Detrick” -----------
--------------------- --- Thursday, March 14,
2024
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