- 4153 - COSMIC INFLATION - the universe is expanding? Scientists are reporting the first evidence that our Earth and the universe around us are awash in a background of spacetime undulations called “gravitational waves”. The waves oscillate very slowly over years and even decades and are thought to originate primarily from pairs of supermassive black holes leisurely spiraling together before they merge.
-------------- 4153 - COSMIC INFLATION - the universe is expanding?
- Radio telescopes monitor 68 dead stars,
called ”pulsars”, in the sky. The pulsars acted like a network of buoys bobbing
on a slow-rolling sea of gravitational waves.
This
effect of the gravitational waves on the pulsars is extremely weak and hard to
detect.
-
- Gravitational waves were first proposed by
Albert Einstein in 1916 but were not directly detected until about 100 years
later when LIGO picked up the waves from
a pair of distant colliding black holes. LIGO detects gravitational waves that
are much higher in frequency than those registered by NANOGrav (NANOGrav's name
comes from the fact that it detects lower-frequency gravitational waves in the
nanohertz range., one cycle every few years).
-
- Higher-frequency gravitational waves come
from smaller pairs of black holes zipping around each other rapidly in the
final seconds before they collide, while the lower-frequency waves are thought
to be generated by huge black holes at the hearts of galaxies, up to billions
of times the mass of our sun, that orbit around each other slowly and have
millions of years to go before they merge.
-
- NANOGrav has picked up a collective hum of
gravitational waves from many pairs of merging supermassive black holes
throughout the universe. NANOGrav's
network of pulsars is also known as a “pulsar-timing array”.
-
- The pulsars, formed from the explosions of
massive stars, send out beacons of light that rapidly spin around at very
precise intervals. These are like
lighthouse beacons that sweep by at a regular rate. You can predict the timing
to a level of tens of nanoseconds. They have the same level of precision of
atomic clocks in some cases.
-
- When gravitational waves travel across the
cosmos, they stretch and squeeze the fabric of spacetime very slightly. This
stretching and squeezing can cause the distance between Earth and a given
pulsar to minutely change, which results in delays or advances to the timing of
the pulsars' flashes of light.
-
- Imagine lots of ripples on an ocean from
pairs of supermassive black holes scattered throughout. Now, we're sitting here on Earth, which acts
like a buoy along with the pulsars, and we try to measure how the ripples are
changing and causing the other buoys to move toward and away from us.
-
- To tease out the gravitational-wave
background a multitude of confusing effects, such as the motion of the pulsars,
the perturbations due to the free electrons in our galaxy, the instabilities of
the reference clocks at the radio observatories, and even the precise location
of the center of the solar system.
-
- NANOGrav is an international
collaboration dedicated to exploring the low-frequency gravitational-wave
universe through radio pulsar timing. NANOGrav was founded in October 2007 and
has grown to more than 190 members at more than 70 institutions. In 2015, it
was designated a National Science Foundation (NSF) Physics Frontiers Center.
-
- The rate at which
the universe is expanding, known as the “Hubble constant”, is one of the
fundamental parameters for understanding the evolution and ultimate fate of the
univeerse. 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.
-
- A cosmic speed
limit tells us how fast the universe is expanding, the Hubble constant. The brightnesses 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.
-
-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, as seen in the right side of the diagram. 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.
-
- A major
justification for building the Hubble Space Telescope was to solve this
problem. 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. 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).
-
- Unfortunately,
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.
-
- However, 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
involves observing Cepheids in a galaxy with a known, geometric distance that
allows us to calibrate the true luminosity of Cepheids. For our program that
galaxy is NGC 4258. 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."
-
- Astronomers
recently got our first Webb measurements from steps one and two which allows
them to complete the distance ladder and compare to the previous measurements
with 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, albeit
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 differences
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.
-
- 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.
-
-
September 16,
2023 COSMIC INFLATION
- the universe is expanding? 4153
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