All Questions
45
questions
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34
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Seyfert Galaxies: How does this statistical deduction about the age of their nucleus make sense?
As per this book, An Introduction to Active Galactic Nuclei by Bradley Peterson:
The nuclear emission must last more than $10^8$ years, because Seyfert galaxies constitute about 1 in 100 spiral ...
- 11
3
votes
2
answers
77
views
How deep can a 'deep field' image be?
Hubble's famous deep field image was created by pointing the telescope at the same spot for 10 days continously. This aggregates photons and creates a coherent image - If I understand what's going on ...
1
vote
1
answer
277
views
What is the average prevalence/density of stars in space?
By average density $D$, I mean that, if you look at a huge volume $V$, you would expect to see $V\cdot D$ stars in that volume.
Just a rough answer is fine. I'm sure the prevalence of stars changes ...
- 1,090
4
votes
2
answers
98
views
At what average rate do supernovas happen?
By average rate $R$, I mean I want an answer with units $$[R]=\frac{\text{supernovas}}{\text{length}^3 \cdot \text{time}}$$. That is to say, if I consider a huge volume $V$ and a long timeframe $T$, I ...
- 1,090
1
vote
0
answers
101
views
Continuity equation for the estimation of cosmological dimensionless parameters
I want to set up a system of equations to find the dimensionless density parameters $\Omega_i$ as a function of $N=\ln(a)$, from the continuity equation:
$$\dot{\rho_i}+3H(p_i+\rho_i)=0$$
where the ...
0
votes
0
answers
125
views
Temperature of Closed Universe in future
This is a question from "Introduction to Modern Cosmology" by Andrew Liddle:
Suppose we live in a closed universe(k > 0), which will recollapse some time in the future. What will the ...
- 77
1
vote
1
answer
125
views
Does the Hubble constant measured for huge distances mean it is not the actual Hubble constant but a very old one?
I am citing Wikipedia about Hubble constant....
"Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value ...
- 2,255
0
votes
2
answers
152
views
What does cosmological redshift say about speed of separation and why?
I've been thinking about cosmological redshift lately and in particular its effect on the apparent speed of separation. In particular, let's imagine a source which emits light pulses with a period of $...
- 485
2
votes
3
answers
144
views
Does light from source one mil. light years away take more than 1 mil. years to reach us?
Because of the expansion of space itself, did light from a galaxy one million light years away actually take more than one million years to reach us?
Also, if we are looking at a galaxy one billion ...
- 4,509
0
votes
1
answer
54
views
Observational evidences of the early universe other than CMB
CMB is a great experimental evidence that the universe was once in an hot themal phase at early times. Supposing all the CMB observations failed for tecnical reason (for example the rocket shipping ...
- 505
0
votes
1
answer
105
views
A thought experiment related to the age of the universe
Okay,
So, this thought experiment starts with the movie, Interstellar. In which humans went through a wormhole to a system consisting of a Black Hole. They landed on a planet (I don't remember its ...
- 13
0
votes
0
answers
114
views
What is the co-moving distance to a point, at which the late-time acceleration of the universe began?
First of all sorry for my English - it is not my native language.
According to the following wonderful diagram made by Pulsar,
the late-time acceleration of the universe began at $t_{acc}=$ 7.7 ...
- 85
12
votes
1
answer
385
views
Is the universe a giant telescope?
Due to the space expansion, the universe should act as a giant telescope. The farther objects are from us (beyond the redshift of $z\approx 1.5$), the larger (not smaller!) they should appear in the ...
- 12.7k
1
vote
1
answer
60
views
Effect of local reference frame and rotation of Universe in CMBR anisotropy
In Kolb and Turner's Early Universe, (see here) it is mentioned that
Variation in the CMBR temperature in different directions is expected due to several effects: the motion of our local reference ...
- 26.8k
0
votes
1
answer
38
views
Measuring temperature of a solar object at different distances to its core
I recently was taught about the composition of the Sun, and that the temperature we know about it is its surface temperature. I find it hard to accept this.
There has to be a way in which ...
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