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# Questions tagged [hubble-constant]

Questions about the Hubble constant, the constant factor of the rate of expansion in the universe

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1 answer
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### How low would the mass of a sphere of uniform density 1 megaparsec in diameter need to be before the expansion of the universe could tear it apart?

Would the mass be too low to be a galaxy or cluster for example?
8 votes
1 answer
1k views

### With 42 supernovae in 37 galaxies, how do we know SH0ES results is robust?

SH0ES is a collaboration that's anchoring the cosmic distance ladder by matching Cepheid data with Type 1a supernovae. Both of these are standard candles, so if we know the distance to a particular ...
• 4,831
2 votes
1 answer
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### What exactly does $k\sigma$ mean in astrophysics and cosmology?

In statistics, $\sigma$ or $1\sigma$ is the standard deviation. For the normal distribution, approximately $68\%$ of the values lie within $1\sigma$ range, $95\%$ within $2\sigma$, and $99.7\%$ within ...
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4 votes
1 answer
97 views

### How could a new cosmological model solve the hubble tension?

So basically, I have to make this presentation about the hubble tension, but I discovered that new hypothetical solutions have been found, like the MoND model or better calibrations for the cepheids ...
2 votes
0 answers
86 views

### Would we know if the universe was rotating?

I was wondering what we would observe if the universe had a small rotation. My conclusion was that galaxies on the equatorial plane of the universe would be slightly more redshifted than galaxies than ...
• 373
7 votes
3 answers
2k views

### How do we work out the light travel time on a cosmic scale?

I just read this article in the AUSTRALIAN SKY & TELESCOPE magazine, Nov/Dec 2022 Issue 140, on P16, KEEP YOUR DISTANCE: How far away are the objects we see in the universe? And on P23: "And ...
• 171
-2 votes
1 answer
171 views

### Why is v lowercase and D uppercase in $v=H_0D$?

Why is v almost always written in lowercase and D in uppercase in $v=H_0D$? OK, v is in lowercase, as usual, but then why is D in uppercase? What's so different/special about it? In my physics school ...
• 171
1 vote
2 answers
1k views

### How can Hubble's original data show that the universe is expanding?

The diagram from Hubble's original data only goes from 0 to 2 Mpc (see here), and yet our Local Group is not supposed to be expanding and it is 1.5 Mpc in radius. How can the data show that the ...
• 171
3 votes
2 answers
189 views

### What is Hubble's parameter/constant H0 supposed to mean, exactly? [duplicate]

Surely it must mean that every second each Mpc expands by that much. Well what else, could it be. So if the expansion is accelerating, then how can H0 be decreasing!? Let's look at Hubble's law: v=H0D,...
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1 vote
1 answer
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### Why is the Hubble time same with age of the universe assuming ideally constant expansion?

Practically, the Hubble time does not perfectly same with age of the universe. But that's considered to coincide when the rate of expansion is constant. However, the constant rate does not mean ...
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7 votes
2 answers
2k views

### How long has it been since dark energy started to reverse our slowdown and accelerate the expansion of the universe? 4 billion years ago? 4.5? 5?

Or has it been as long as six billion years? Over the last few years I keep seeing longer and longer numbers... Have researchers noticed something different recently? Or found a mistake in their ...
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4 votes
1 answer
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### What's the relation between the parallax distance and the luminosity distance?

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 ...
• 143
6 votes
0 answers
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### Hubble tension and Cepheid systematic error

The Hubble tension is the discrepancy between measurements of the local rate of expansion of the universe vs the predicted rate inferred from CMB and the Friedmann equations. A dizzying array of ...
1 vote
1 answer
136 views

### Coasting universe Hubble parameter behaviour

I’ve recently been trying to wrap my head around the potential behaviours of the Hubble parameter over time for various cosmological models but I’ve run into a little snag when it comes to a coasting ...
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3 votes
0 answers
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### Kinetic energy in cosmology?

Spacetime expands at an accelerated rate due to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy ...
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11 votes
1 answer
2k views

### Can the Hubble constant be measured directly?

By my calculations, the expansion of the universe should cause LIGO’s interferometers to alternate between constructive interference and destructive interference every couple days. Is this a practical ...
3 votes
0 answers
69 views

• 141
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1 answer
370 views

### When was Hubble tension first noticed? When was this term first used?

When was Hubble tension first noticed? When was this term first used? Who used this term for the first time?
0 votes
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38 views

### Must all galaxies in a galaxy cluster have the same expansion rate?

If we observe a distant galaxies cluster where all galaxies travel with the same speed away from us, except for one galaxy whose speed is significantly lower than of the neighboring galaxies. Does ...
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