# Questions tagged [hubble-constant]

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

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### 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 ...
1 vote
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### 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' ...
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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 ...
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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 ...
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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 ...
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### What would have been the case if the Hubble diagram was linear throughout?

Redshifts are converted into velocities using v = cz; this is a good approximation for low redshifts over which the Hubble diagram is linear. Since the Hubble diagram deviates from linearity at high ...
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### 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. ...
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1 vote
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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 ...
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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 ...
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 ...
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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 ...
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### 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 ...
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1 vote
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### 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 ...
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### 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 ...
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### 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 ...
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### 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....
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### 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 ...
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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) ...
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### 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 $H(t)$ ...
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### 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 ...
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### 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 ...
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1 vote
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### 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 ...
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### 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 ...
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### 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(...
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### 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. ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ... 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 ...
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### 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 ...
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### '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 ...
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### 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?
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### 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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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### Relationship between Hubble paramater and absolute magnitude

I have a set of data that is absolute magnitude of a star and its redshift. Is there any way to compute the value of the hubble parameter given that magnitude and redshift?
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### Where a database of different z values of redshift can be found?

I was working on reshift details for various galaxies but do not know any database to find the z values given to redshift galaxies. Can anyone help?
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### Where can I find a database of galactic spectra?

I was wondering where spectra of galaxies at various redshifts are to be found. I'm looking for ones that can be used to find the recessional velocity of the galaxy and eventually the Hubble Constant. ...
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### How is the expansion rate of the universe obtained from Baryon Acoustic Oscillations?

I understand that we can measure the sound horizon from the anisotropies in the CMB. Large galaxy surveys today can construct two-point correlation functions, which reveal peaks at distances equal to ...
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### Could Hubble's Law simply be light's frequency decaying over time? [duplicate]

Is it proven that the frequency of EMR does not decay (at all, ever), and if not, could the observations that led to Hubble's Law simply be that the frequency of light decays over very large durations/...
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### Are there recommendations to use "Hubble-Lemaître constant" instead of "Hubble constant"?

The IAU recommends renaming the Hubble law as the Hubble-Lemaître law, as discussed at its General Assembly in 2018 (see this press release and the links therein). I have now stumbled upon an ...
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### how do we know the universe is STILL expanding?

According to Hubble more distant galaxies are moving away faster but since the light took billions of years to reach us we should say they WERE moving away faster billions of years ago. How do we know ...
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### How to calculate how much a galaxy moves from its coordinate at redshift 0 to redshift 1?

I have two simulation snapshots in my hand at redshift 0 and 1. I know x, y, z coordinates of the galaxies in both redshifts 0 and 1, however there is no way for me to identify a single galaxy in both ...
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### Light beam (1 photon) in the limited universe?

If our universe is not infinite, what happens with the light beam (or photon) when it will travel through the whole universe? For example, observable universe according to wikipedia has diameter 93 ...
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