- 4199 - GRAVITY - which theory works universally? Techniques have been developed to split galaxies into different populations and keep track of swaps between them. In time, new technologies will be able to detect the tiny effects of gravitational redshift, and consequently distinguish a modification of Euler's equation for dark matter from a modification of Einstein's equation for gravity.
--------------------- 4199 - GRAVITY - which theory works universally?
- We all know that
time moves faster when you don't want it to and slower when you do. Gravity also causes distortions in
time. Distortion of time helps tell
“modified gravity” apart from a dark force.
What is the “dark force”?
-
- With his theory of
General Relativity in 1915, Albert Einstein revolutionized how we think about
our universe. Rather than the cosmos simply providing the room for the planets
and stars to orbit each other, “space and time” themselves were now dynamical
entities in one ever-evolving play with “matter and light”.
-
- Einstein's
equations described how stars, galaxies and all other matter curve or warp
“space and time”. The galaxies and the light rays then travel in this distorted
space-time according to the equation provided by the 18th-century Swiss
mathematician Leonhard Euler.
-
- Can we
differentiate a universe where Einstein's equations were violated from a
universe where Euler's equation were modified?
If what we observed with telescopes disagreed with what Einstein and
Euler prescribed, would we be able to tell which one of the two was wrong or
right?
-
- Existing
observations have beautifully confirmed the validity of both their theories.
The reason to put those to the test comes from the fact that our universe is
filled with unknowns.
-
- In the 1930s, the
Swiss-American astrophysicist Fritz Zwicky observed that there was five times
more matter in the universe than we can detect with our telescopes. He called
this new matter "dark matter."
-
- Nearly 100 years
later, we still don't know what dark matter is. We have never detected a
particle of dark matter and we don't know how it moves. It is therefore
legitimate to question if it behaves as ordinary matter and obeys Euler's law.
Could it be affected by other forces and interactions, which would change the
Euler equation?
-
- In 1998, two
groups of astrophysicists observed that the expansion of our universe is
accelerating, contrary to the deceleration expected because of the
gravitational attraction between galaxies.
-
- We don't know what
causes this strange in accelerating behavior?
Is it due to the presence of yet another "dark" substance that
has repulsive gravity? Or is it due to gravity itself, meaning Einstein's predictions
of how it behaves over very large distances would be wrong? Testing Einstein's
and Euler's equations is therefore the logical consequence of these mysteries.
-
- Checking if
Einstein's gravity works over the vast distances of the universe has become an
active field of research. Theoreticians propose new ideas for how gravity could
work differently, while astronomers use increasingly advanced facilities to
provide the data needed to test their theories.
-
- Researchers have
identified a particular "smoking gun" signature of modified gravity
known as the "gravitational slip." General Relativity predicts that
the pathways of light and matter should bend in the same way when traveling
through the same distorted space-time.
-
- This is much like
the fact that different objects fall at the same rate in Earth's gravity (if
the air resistance could be neglected).
Something Galileo famously demonstrated from the tower of Pisa. By
comparing the way galaxies fall into gravitational wells to how the light from
these galaxies is deflected by gravitational lensing, we can deduce if they
feel the same gravity.
-
- If different, we
would say there was a “gravitational slip”. Measuring the slip is one of the
main targets of “Euclid, a wide-angle space telescope” launched by the European
Space Agency on a Space-X rocket.
-
- But what if Euclid
found that there was a slip? Could we be certain that it occurs due to a
modification of gravity, or could it also be due to a modification of Euler's
equation? The latter would be different if
the dark matter in the galaxies were subject to a new force.
-
- Despite the common
expectation, measuring the gravitational slip would not allow one to
distinguish a modification of Einstein's laws from a modification of Euler's
equation.
-
- However, the
distinction may be possible if one could measure the another effect called
"gravitational redshift," which should be possible with telescopes
such as the “Dark Energy Spectroscopic Instrument” and the upcoming “Square
Kilometer Array”.
-
- If the measured
gravitational slip signals a breakdown of General Relativity we would need to
measure the velocity of normal matter when it is not confined to a galaxy. In
practice, however, we can only observe the light from stars that reside in
galaxies, and hence move together with the dark matter.
-
- Telescopes can only
measure the collective motion of a galaxy that contains both normal matter and
dark matter. So, if a galaxy were to fall into a gravitational potential in a
way that was not consistent with our expectations, we would be unable to tell
if it's because the dark matter is doing something, or because gravity was
modified.
-
- There is a way to
probe the gravitational potential directly through the way it distorts “time”
via gravitational redshift. The time
kept by a clock that's on top of a tall mountain is different from that of a
clock at the sea level. These differences are extremely tiny but are, in fact,
very important in the design of satellite navigation systems. That is the way satellites can manage cell
phones and provide navigation aids.
-
- When the light from
a galaxy escapes the gravitational potential it is falling into, its color
shifts closer to red. This gravitational redshift is solely due to time
distortion. Gravitational lensing, which differs from redshift, is due to both
space and time distortions, as opposed to just time alone.
-
- We need to have
both lensing and redshift in order to isolate the gravitational slip. It is
this ability to separate the distortion of space and time from the distortion
of time alone that is key to measuring true gravitational slip.
-
- A measurement of
the gravitational redshift is impossible if we cannot easily keep track if a
pair of galaxies swapped their positions. While it's not that hard to tell any
two galaxies measured by a telescope apart, when running a statistical analysis
on a catalog of millions of galaxies, you can quickly lose the ability to
assign any identity to the galaxies; at some point they are all treated as
points on the sky.
-
- Which theory pans
out or does a new one replace them? Only
time will tell!
-
-
October 24, 2023 GRAVITY -
which theory works universally
4199
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