- 4143 - GRAVITATIONAL WAVES - tell us about the Universe - 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.
----------- 4143 - GRAVITATIONAL WAVES - tell us about the Univers
- Astronomers monitored 68 dead stars, called
“pulsars”. 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.
-
- Astronomers have a new way of probing what
happens as monstrous black holes at the cores of galaxies begin a slow but
inexorable death spiral.
-
- 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 lumber around each other slowly and have
millions of years to go before they merge.
-
- NANOGrav is thought to have 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, which 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.
-
- To search for the background hum of
gravitational waves, the team developed software programs to compare the timing
of pairs of pulsars in their network. Gravitational waves will shift this
timing to different degrees depending on how close the pulsars are on the sky.
-
- 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, we had to nail down 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.
-
- Astronomers have tried to find merging
supermassive black holes with telescopes for years. They are getting closer and finding more
candidates, but because the black holes are so close together, they are hard to
distinguish. Having gravitational waves as a new tool will help better
understand these beasts.
-
- LIGO has observed gravitational waves from
two black holes that orbited each other and then merged to form a bigger black
hole. The final black hole had a mass 60 times that of our Sun.
-
- This event occurred around a billion light
years away from Earth. The merger was extremely energetic (for a fraction of a
seconds the event released 50 times more energy in gravitational waves than the
all the stars in the entire Universe in light), but by the time the waves
reached us, they were so weak that the change in the length of LIGO's arms was
less than a 1,000th of the diameter of the core of an atom. That's tiny.
-
- This tells us that binary black holes do
exist. It also tells us that they form, evolve and die during a period shorter
than the age of the Universe. We’ve never seen binary black holes before. We've
never found black holes of this mass before. It looks like these mergers should
be common enough that we will see more in future observations with LIGO. Then
we can start to understand exactly what is out there and how these binaries are
made.
-
- General relativity is our best theory of
gravity. In general relativity, gravity can be thought of as the effect of the
curvature of spacetime. Massive objects bend space and time; the curvature in
spacetime changes how things move.
Gravitational waves are ripples in spacetime.
-
- When objects move, the curvature of
spacetime changes and these changes move outwards (like ripples on a pond) as
gravitational waves. A gravitational wave is a stretch and squash of space and
so can be found by measuring the change in length between two objects.
-
- In our everyday lives we think of
three-dimensional space (up/down, left/right, forward/back) and time as
completely separate things. But Einstein’s theory of special relativity showed
that the three spacial dimensions plus time are actually just part of the same
thing: the four dimesions of spacetime.
-
- In general relativity, Einstein went further.
Not only are space and time part of the same thing, but they are both warped by
mass or energy, causing a curved spacetime. Things like to move along the
shortest route available; when spacetime is flat, this looks like a straight
line. But when spacetime is warped, the shortest route might not look straight
anymore.
-
- When you are flying over the curved earth,
your airplane’s flight path will look curved, even if you are going “straight”
from A to B. We can see and measure the effect of curved spacetime. The sun’s mass curves spacetime so the Earth
moves in a circular orbit around the sun.
-
- It is hard to imagine a four-dimensional
spacetime, let alone what a curved version of this looks like, so we often
simplify this by thinking of an example in two dimensions. We can imagine a
two-dimensional spacetime as a rubber sheet; dropping a heavy object on the
sheet will bend and distort the sheet. In a similar way, mass or energy
distorts spacetime around it.
-
- Black holes are the regions of strongest
gravity in the Universe. They are where the curvature of spacetime is so steep
that all paths lead inwards. Eventually nothing can climb up the curvature no
matter how fast it goes; even light, the fastest thing in the universe, can’t
escape if it gets too close to a black hole.
-
- Watching binary pulsars the orbit of the
binary will shrink by the amount predicted by gravitational waves emission, but
we don't see the waves themselves. Measuring the waves themselves would be the
final piece of evidence for the predictions of Einstein's general relativity.
-
- Neutron stars are old, dead stars that
collapsed down to an extremely dense object. Roughly the mass of our sun
compressed into about the size of a city. Pulsars are rotating neutron stars
which emit a beam of radiation. As the pulsar rotates, the beam of radiation
sweeps across the Earth like a cosmic lighthouse. A binary pulsar is where a
pulsar orbits another star or sometimes another pulsar.
-
- Gravitational waves are a new way of
observing the Universe. Astronomy traditionally uses light to explore the
cosmos, but there are lots of things you can miss because a lot of the universe
is dark, including black holes.
-
- The Laser Interferometer Gravitational-Wave
Observatory (LIGO) is made up of two gravitational wave detectors in the USA
designed and operated by Caltech and MIT. In addition the LIGO Scientific
Collaboration with a 1000 scientists from around the world provides crucial
support for the LIGO science from instrument development to data analysis and
astronomy.
-
- One LIGO observatory is located in
Livingston, Louisiana and the other in Hanford, Washington. Each observatory
contains an enormous, extremely sensitive laser ruler. We bounce lasers along
two 4-kilometre long paths, or “arms”, which are at right angles to each, and
then compare the length of each path.
-
- A gravitational wave can change the length
of the arms, but the effect is extremely small (one part in
1,000,000,000,000,000,000,000 for the strongest waves), so the instruments need
to be extremely sensitive, which became possible using completely new
technologies and a new interferometer concept.
-
- LIGO has just finished its first
observations using its new “advanced” sensitivity. It will slowly be improved
over the coming years, making it even more sensitive. Is joined by Virgo, a
detector in Italy. There is also another detector being built underground in
Japan called KAGRA. There is also a plan for putting a LIGO detector in India.
-
- Further in the future, there will be a
space-based mission called eLISA. This will be much bigger (100 times the size
of the Earth) and look for gravitational waves from much more massive objects.
-
-
September 5, 2023 GRAVITATIONAL WAVES
- about the Universe 4143
------------------------------------------------------------------------------------------
-------- 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” -----------
--------------------- ---
Saturday, September 9, 2023 ---------------------------------
No comments:
Post a Comment