- 4420 - UNIVERSE EXPANSION RATE? - how fast is it, really? - The notion of the absolute speed limit comes from special relativity, but who ever said that special relativity should apply to things on the other side of the universe? That's the domain of a more general theory. A theory called “general relativity”.
------------- 4420 - UNIVERSE EXPANSION RATE? - how fast is it, really?
- One of the great unsolved mysteries of
cosmology is known as the “Hubble tension”. It stems from our inability to pin
down the precise rate of the universe expansion. There are several ways to
calculate this expansion, from observing distant supernovae to measuring the
Doppler shift of maser light near supermassive black holes, and they all give
slightly different results.
-
- Maybe we don’t fully understand the
structure of the Universe, or maybe our view of the heavens is biased given
that we are located deep within a galactic supercluster. The bias problem is even worse than we
thought.
-
- If we were floating deep in space, far away
from any galaxies, then our view of cosmic expansion would be free of
gravitational influences and we could better see how distant galaxies move away
from us. Since we are part of a local cluster of galaxies, we have a bit of
bias in our data. This is why many local galaxies are “blue-shifted”.
-
- The Universe isn’t contracting near us,
we’re just in a galactic gravitational well. We can easily account for this
bias, so it isn’t a problem. However, given the Hubble tension, a team recently
looked for gravitational biases beyond the local group, hoping it would solve
the issue.
-
- They looked at the largest gravitational
structure we’re a part of, known as the “Laniakea Supercluster”. It is a
massive cluster of galaxies more than 520 million light-years across,
containing more than 100,000 galaxies, including the Milky Way. Our local group
is being pulled toward the heart of Laniakea, and thus our motion through the
Universe could skew our observations of cosmic expansion.
-
- When the team measured the gravitational
influence of the supercluster as a whole, they found it does bias our
observations by about 2% – 3%. But it’s biased in the wrong direction. In other
words, by not taking the effect of Laniakea into account, the Hubble tension
seemed smaller than it actually is. These new results show that the tension is
2% – 3% greater than we thought.
-
- Removing subtle biases from our cosmological
data is challenging, so it is possible that further observations may swing the
results back in the right direction. But we can’t rely on bias alone to solve
this mystery. Clearly, something subtle and strange is going on, and the
solution isn’t obvious. It will take a great deal more study to understand
“Hubble’s tension”.
-
- How can the universe expand faster than
light travels? It seems like it should
be illegal, doesn’t it? Over and over (and over and over) we're told the
supreme iron law of the universe: Nothing — absolutely nothing — can go faster
than the speed of light.
-
- Some galaxies are moving away from us faster
than the speed of light. It's important
to note that we live in an expanding universe. Every day the galaxies get
farther apart from each other, on average. There are slight motions on top of
that general expansion, leading to instances such as the Andromeda Galaxy
heading on a collision course for the Milky Way. But in general, in the biggest
of pictures, the galaxies are getting farther away from each other.
-
- A key feature of this expansion is how
uniform it is. The objects close to you
appear to move away with some speed, but the farther objects would appear to
move faster. Even though they are moving at a constant speed, the apparent
speed changes with distance.
-
- The modern value is 68 kilometers per
second per megaparsec, plus or minus a couple.
One megaparsec is 1 million parsec, which is 3.26 million light-years.
-
- A galaxy 1 megaparsec away will appear to
be receding away from us at 68 kilopmeters / second. If you look at a galaxy 2
megaparsec away, it recedes at 136 km/s. Three megaparsec away? You got it! 204
km/s. And on and on: for every megaparsec, you can add 68 km/s to the velocity
of the far-away galaxy.
-
- At some point, at some obscene distance, the
speed tips over the scales and exceeds the speed of light, all from the
natural, regular expansion of space.
-
- It's true that in “special relativity”,
nothing can move faster than light. But special relativity is a local law of
physics. Or in other words, it's a law of local physics. That means that you
will never, ever watch a rocket ship blast by your face faster than the speed
of light. Local motion, local laws.
-
- But a galaxy on the far side of the
universe? That's the domain of general relativity, and general relativity says
that a galaxy can have any speed it wants.
It goes deeper than this. Concepts like a well-defined
"velocity" make sense only in local regions of space. You can only
measure something's velocity and actually call it a "velocity" when
it's nearby and when the rules of special relativity apply.
-
- Stuff super-duper far away, like the
galaxies we're talking about it? If it's not close, it doesn't count as a
“velocity” in the way that special relativity cares about. Special relativity doesn't care about the
speed, superluminal or otherwise
-
-
April 4, 2023 UNIVERSE EXPANSION
RATE? - how fast is it, really? 4420
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--------------------- --- Friday, April 5, 2024
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