All Questions
Tagged with astronomy space-expansion
52
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129
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Is cosmological redshift associated with recession velocity when the light left or when it arrived?
Is the cosmological redshift $z$ associated with the recession velocity when the light left, when it arrived, or something in-between?
user300520
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103
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Measured Value of Hubble's Constant [duplicate]
As we know, Hubble's Constant is not exactly a constant, but its value varies with time. However, we also speak of measuring its present-day value. The measurements by cosmic distance ladder would ...
- 442
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Andromeda Galaxy and Hubble
When Hubble determined the distance to Andromeda he based his estimation on Cepheids. However, the result was less than half the current value. What was the cause of this error and could another ...
- 167
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What's the relation between the parallax distance and the luminosity distance? [closed]
i have read that Riess and his team are able to measure $H_0$ from supernovae calibrated using Cepheid in a model independent way.
from what i have gathered they find the absolute luminosity of ...
- 299
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Do most distant objects on the Hubble diagram 'tell' as the Hubble constant has been valid for at least 60 million years?
As we all know the deeper we look into the space the further we look into the past... So if the Hubble diagram shows the most distant objects at a 20 Mpsec or ~60Mly distance from us (and as for not ...
- 2,255
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Does only the space region movement due universe expansion affect the possibility of viewing a star?
My question is maybe simple and easy to answer because the intrinsic speed of the star affects only the final frequency of observed light but are there any tricks about the combination of object ...
- 467
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Calculating distance to galaxies seen through a telescope
When we see a galaxy through a telescope, say the James Webb telescope, it is said we see a galaxy as much as about 10 billion light years away. But 10 billion years ago we were much closer (because ...
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113
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If there was an infinite amount of matter at big bang to form an infinite universe how it was possible for a so huge amount of matter to expand?
If there was an infinite amount of matter at big bang to form an infinite universe how it was possible for a so huge amount of matter to expand? Is it better to think that the universe is spatially ...
- 467
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When we measure our distance from a very distant astrophysical object image do we input its past location and our present location?
When we measure our distance from a very distant astrophysical object do we input its past location and our present location? Please take a look on the picture where our past location is A1 and the ...
- 1,815
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If Andromeda galaxy is 2.5 million light years away, shouldn't the light we see be younger than 2.5 million years?
I was wondering about this. I've always heard that (for example) Andromeda being 2.5 million light years away means that what we see is from 2.5 million years ago. However, taking into account the ...
- 133
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Accelerated expansion of the universe caused by dark energy
Maybe I am misunderstanding this concept but my question is the following: would we state that the universe is accelerating its expansion if we measure the speed of an astrophysical object when it ...
- 2,255
4
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What are some values for the scale factor $a(t)$ in cosmology?
In cosmology, the scale factor is given in $D(t)=a(t)D_0$, where $D(t)$ is the proper distance at epoch $t$ and $D_0$ is the distance at reference time $t_0$.
What are some of the values that $a(t)$ ...
- 51
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69
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Four Momentum in an Isotropic and Homogeneous Universe
We define the four-momentum of a particle by $p^\mu = (E, \mathbf{p})$, where $E$ is energy and $\mathbf{p}$ the three momentum of the particle. In cosmology, we conventionally use the Friedmann-...
- 201
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67
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If a very distant supernova has a high Doppler effect redshift do our eyes see its evolution picture like in 'slow motion'?
If a very distant supernova has a high Doppler effect redshift do our eyes see its evolution picture like in 'slow motion'? If a same type of supernova apparently lasts longer if it has more ...
- 1,815
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Negative mass density in cosmology?
I came across a problem in Chapter 6 of An Introduction to Modern Cosmology by Andrew Liddle that reads:
Identify a sufficient and necessary condition that must be satisfied by the equation of state ...
- 77