Questions tagged [hubble-constant]

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

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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 ...
Curious Cat's user avatar
-2 votes
1 answer
159 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 ...
Curious Cat's user avatar
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 ...
Curious Cat's user avatar
3 votes
2 answers
183 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,...
Curious Cat's user avatar
1 vote
1 answer
73 views

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 ...
XX X's user avatar
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2 answers
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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 ...
Kurt Hikes's user avatar
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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 ...
Alucard's user avatar
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6 votes
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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 ...
Kevin Kostlan's user avatar
1 vote
1 answer
109 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 ...
Scott's user avatar
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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 ...
vengaq's user avatar
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11 votes
1 answer
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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 ...
Spencer Joplin's user avatar
3 votes
0 answers
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Critical density and Hubble parameter

Is it possible for the Hubble parameter of an /expanding/ universe made of radiation, matter and cosmological constant to be increasing with time? I'm trying to figure out if any form of scale factor $...
ABC's user avatar
  • 53
4 votes
1 answer
242 views

Why is the Hubble parameter constant for an accelerating universe?

The Hubble parameter is thought to become constant and to attain a value of about 60 km/s per parsec. So every second a parsec of space grows with 60km. Is this consistent with an accelerated ...
Felicia's user avatar
1 vote
2 answers
160 views

Since the Hubble Constant changes over time (it's a variable parameter), why can't the conflicting values of 67.4 and 73 both be right?

Are the conflicting values from the 'early universe' (Planck) method and the 'late universe' (Distance ladder) method actually compatible? Since during the latter period of the universe 'dark energy' ...
Kurt Hikes's user avatar
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4 votes
1 answer
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How sure are we about the speed of light in vacuum in deep/far space?

A very naive question related to galaxy recessional velocity. In simple term : The further away it is, the faster it goes. And we know it's redshifting because we can mesure a change in frequency of ...
ker2x's user avatar
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5 votes
7 answers
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Interpretation of Hubble constant in SI units

The standard interpretation of Hubble constant $\approx 70~\text{km/s/Mpc}$ means that each mega-parsec of distance adds $70~\text{km/s}$ to a galaxy recession velocity from us (or to a space ...
Agnius Vasiliauskas's user avatar
17 votes
11 answers
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Is there physical evidence to distinguish between the expansion of space and an anthropocentric universe?

When we look in all directions, we see distant objects red-shifted, with the size of the red-shift correlated with the distance from us. As I understand it, the consensus among cosmologists is that ...
Brionius's user avatar
  • 331
2 votes
1 answer
284 views

Why are galaxies thought to get bigger with distance in an expanding universe model?

In an expanding universe model. In addition to galaxies getting fainter with distance, they are also thought to get bigger. Thus the surface brightness of the object should decrease with distance. ...
Tivity's user avatar
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1 answer
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What exactly is the estimated distance to a far-off object when they say '50 Mpc/h' or '50 Mpc h^-1'? Is it less than 50 Mpc? Do you ÷ by 67 or 74?

Several recent arxiv.org papers I read mention distances to very distant objects in Mpc (megaparsecs) divided by Hubble's 'constant'.... Does that mean we should divide the Mpc or Gpc (gigaparsecs) by ...
Kurt Hikes's user avatar
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4 votes
2 answers
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Hubble constant from GW events

I am trying to get the Hubble constant from gravitational wave events. On the GWOSC website, for all events, redshift and Luminosity distances are mentioned. Now, by this information, I can get my ...
Ashley Chraya's user avatar
1 vote
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Can we observe galaxies after their recession velocity exceeds the speed of light? [duplicate]

It doesn't make sense to me that light could ever reach us from a galaxy moving away from us faster than the speed of light. But this video says that it can happen. Is this true? Could someone ...
Connor Garcia's user avatar
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3 votes
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Calculating the present comoving distance or light travel distance of distant objects when only one value is given?

I was wondering how to calculate either the present comoving distance or the light travel distance when the source material only gives one value? Is there and an online calculator or simple equation ...
Jim Daniels's user avatar
2 votes
1 answer
124 views

Observability of proper distance

Hubble's law at current time is as follows. $$v=H(t_0)r$$ But if you look at the explanation on Wikipedia: Strictly speaking, neither $v$ nor $D$ in the formula are directly observable, because they ...
빛나는밤's user avatar
2 votes
2 answers
205 views

Could the Hubble Constant be an artifact of the structure, and thus a way to directly measure the universes's age?

"Expressed in inverse years, the value 70 km/s/Mpc comes to about one divided by 14 billion years - the approximate age of the universe." Could the Hubble Constant be an artifact of the ...
Mark Besser's user avatar
2 votes
1 answer
449 views

Why would a quantity like the 'Hubble contrast' be squared, then have its square root taken?

From Sabine Hossenfelder's recent video, New Evidence AGAINST Standard Cosmology: And her source.... Figure 2. The variation with increasing void radius of the variance of the Hubble parameter, the ...
Kurt Hikes's user avatar
  • 5,087
9 votes
1 answer
276 views

How distant were the furthest currently-observable cosmic events when their currently-observed radiation was emitted?

(Edited for clarity. Thanks to James K and Connor Garcia.) This question about the most distant, observable cosmic objects made me wonder if we know the distance that was between us and them at the ...
Glycoversi's user avatar
6 votes
2 answers
184 views

How does the hypothesis of the "inconstant Hubble constant" solve the current crisis in cosmology?

It was published in a paper more or less like two months ago. I'd like to know also if more accurate measurements are necessary to close the gap between the model of the universe and the data reported....
bestofthebeast's user avatar
5 votes
1 answer
346 views

If we watched extremely red-shifted galaxies near the edge of the observable universe for a very long time, how would they change? Would more appear?

I have understanding sphere eversion as #1 on my bucket list (if I ever get a round tuit) but understanding metric expansion seems to be a rapidly receding possibility :-) Question: Suppose it takes ...
uhoh's user avatar
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5 votes
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Explain as simply as possible how the Sunyaev-Zeldovich (SZ) effect is used to estimate the Hubble constant

The Sunyaev–Zeldovich effect (SZ effect) is useful in determining the Hubble constant because it is independent of the cosmic distance ladder. This effect occurs when CMB (cosmic microwave background) ...
Astroturf's user avatar
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5 votes
0 answers
78 views

How did pressure evolve in the (early) universe?

I am trying to derive how the cosmological pressure $p(t)$ evolved over time in the universe, especially in the radiation and matter dominated epochs. There are some very nice explanations how $H(t)$ ...
NeStack's user avatar
  • 313
2 votes
1 answer
447 views

Can the Hubble Sphere ever extend over the cosmic event horizon?

I am struggling to reconcile the Hubble volume with the idea of a cosmic event horizon. As I understand, the Hubble volume is increasing over time because Hubble's constant is decreasing. This should ...
Matthew H's user avatar
  • 147
2 votes
0 answers
94 views

How many galaxies are predicted in the observable universe? Does it contain dwarfs? Is there any size-ratio diagram?

According to quite recent research the observable universe contains about 2 trillion galaxies ($2 \cdot 10^{12}$). But what is counted there? Does this number also contain dwarf galaxies? According to ...
J. Doe's user avatar
  • 245
1 vote
0 answers
87 views

Objects beyond 13.3 billion light-years away are (apparently) moving away from us at faster-than-light speeds? [duplicate]

That is, if you use the cosmic distance ladder method, and a value of about 73.5 for the Hubble constant.... But, if you plug in the Planck CMB value of about 67.5, you get a distance of about 14.5 ...
Kurt Hikes's user avatar
  • 5,087
0 votes
0 answers
109 views

What is the age of the universe if you were standing in a galaxy far far away such as GN-Z11?

If we agree with the universe starting from a Big Bang, 13.8 billion years ago, expanding at different rates governed by the Hubble constant of 67.4 km/s/Mpec, somewhere about 13.4 billion light years ...
ParityViolator's user avatar
5 votes
1 answer
141 views

How, exactly, does the precise measurement of the CMB's polarization modes and temperature fluctuations tell us the value of Hubble's 'constant'?

Amidst all the talk a year and change ago about the value of the Hubble parameter reached by the Planck satellite team, and how it's value differed from the value reached by the 'distance-ladder' team(...
Kurt Hikes's user avatar
  • 5,087
2 votes
0 answers
181 views

Clarifications about distances in cosmology

I would like to get clarifications about some usual notions of distances in cosmology. First, is the comoving distance the current distance of objects whose light has been reached by us now, i.e. ...
user avatar
0 votes
2 answers
89 views

is there a way to determine a distant galaxy’s speed relative to the Hubble Flow by measuring time dilation effects between there and this galaxy?

As I understand it there is a preferred frame of reference based on the velocity for the CMB and our galaxy is moving in relation to it (at about 600 km/s). I think this is how the Hubble Flow is ...
ParityViolator's user avatar
3 votes
1 answer
170 views

What was the the value of the Hubble constant at the time of the CMB's 'release' (i.e., 379,000 years after Big Bang)?

What about its value roughly 9 billion years after the Big Bang, when dark energy started to 'take over' and accelerate the expansion of the universe? Is there a timeline or chart somewhere that shows ...
Kurt Hikes's user avatar
  • 5,087
0 votes
1 answer
75 views

Which redshift is used to determine the Hubbleconstant?

I think they measure cosmological redshift to use in the Law of Hubble-Lemaître together with the distance to calculate $H_0$. Is this correct, or do they use Doppler shift (too)? $H_0$ indicates how ...
PrincepsMaximus's user avatar
3 votes
3 answers
228 views

Intuition about Hubble's Law

I have recently learnt about Hubble's Law and this is my understanding of it: Galaxies move away from us at a rate that is proportional to their distance from us. So, considering an arbitrary unit of ...
user avatar
5 votes
3 answers
1k views

Does a merging massive binary black hole ‘emits’ more than one gravitational wave?

If we want to use gravitational waves (GW) to determine the Hubble constant, we need to find the source in the electromagnetic spectrum (EMS). However, we need to be lucky to ‘see’ it simultaneously ...
PrincepsMaximus's user avatar
14 votes
1 answer
1k views

How is the Hubble constant determined from gravitational waves?

We know there is a discrepancy between measurements of the Hubble constant, $H_0$. On one side there is the method of the Planck mission, where they use the CMB and the $\Lambda$CDM model to determine ...
PrincepsMaximus's user avatar
4 votes
2 answers
686 views

'Little h' usage in cosmological simulations

I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
gabe's user avatar
  • 141
5 votes
1 answer
345 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?
Youngsub Yoon's user avatar
0 votes
0 answers
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 ...
Yos's user avatar
  • 131
2 votes
0 answers
71 views

Catching up with Hubble domain galaxy at constant acceleration: is it possible? If yes, what time will it be when I arrive?

An elf gave me a magic spaceship for Christmas: it can maintain a thrust of $\alpha = 10\ \mathrm{N/kg}$ indefinitely. Otherwise, I'm still constrained by physics. I plan to set out on a journey into ...
Alexander Klauer's user avatar
0 votes
1 answer
146 views

Why is constant velocity drift not an accepted explanation of Hubble expansion?

Edward A Milne at the Oxford University proposed that what Hubble was observing was simply a natural sorting of random galactic motions. If a group of galaxies formed together moving at various ...
Ritesh Singh's user avatar
2 votes
2 answers
206 views

Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant?

The Supernova 2006cm has a redshift of 0.0153 which translates into a recession speed of 4600 km/s. It has a distance modulus of 34.71 which translates into a luminosity distance of 87 Mpc. This ...
Ritesh Singh's user avatar
2 votes
1 answer
230 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=v/c$ won't apply)? [The ...
Ritesh Singh's user avatar
13 votes
1 answer
423 views

The recent results on Hubble constant measurements

In recent news there is this announcement In the introduction they say: Distance measurement discrepancy: a $4.4σ$ tension on the value of $H_0$ and I understand the discrepancy is with the value ...
anna v's user avatar
  • 233