What is Hubble constant: Definition and 72 Discussions

Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words, the farther they are the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the spectrum.
Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model.
The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation v = H0D, with H0 the constant of proportionality—Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation v, i.e. the derivative of proper distance with respect to cosmological time coordinate. (See "Uses of the proper distance" for some discussion of the subtleties of this definition of "velocity".)
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 is about 70 (km/s)/Mpc. However, the SI unit of H0 is simply s−1, and the SI unit for the reciprocal of H0 is simply the second. The reciprocal of H0 is known as the Hubble time. The Hubble constant can also be interpreted as the relative rate of expansion. In this form H0 = 7%/Gyr, meaning that at the current rate of expansion it takes a billion years for an unbound structure to grow by 7%.
Although widely attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if that were the case. Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of, and distance to, distant bodies, and suggested an estimated value for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as the Hubble constant. Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. Though the Hubble constant H0 is roughly constant in the velocity-distance space at any given moment in time, the Hubble parameter H, which the Hubble constant is the current value of, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer.

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  1. H

    Equation to give the lookback time as a function of redshift

    Hi, I'm currently reading the introduction to cosmology second edition by Barbara Ryden and at the page 105, the author says we get ##t_0 - t_e = H_0^{-1}[z - (1 + \frac{q_0}{2})z^2]## by inverting ##z = H_0(t_0 - t_e) + (1 + \frac{q_0}{2}H_0^2(t_0 - t_e)^2)##. However, I can't figure out how...
  2. PhanthomJay

    I Hubble Constant, Dark Energy and the Expanding Universe

    Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now...
  3. M

    A Updated Hubble constant from TRGB measurements

    Scolnic et al have put out a new preprint with an updated value for the Hubble constant as measured from the Tip of the Red Giant Branch: https://arxiv.org/abs/2304.06693
  4. T

    I Question about the Hubble Constant

    If there were humans on Earth 2.5 billion years ago (universe 20% younger) and they had today's equipment , would they measure the Hubble constant smaller, larger or the same?
  5. Buzz Bloom

    I A relationship between flat vs. finite and (2) Hubble constant tension

    One recent example of a thread discussing flat or not is: https://www.physicsforums.com/threads/could-the-universe-be-infinite.1011228/ . I found an interesting 2021 article regarding the Hubble constant tension...
  6. A

    I Is the Hubble constant a constant or is it a parameter?

    (P.S! this is an open question, feel free to answer)
  7. R

    A Hubble Tension and Cosmic Acceleration: A measurement artifact?

    By analyzing 91,742 reported extra-galactic distances and their one sigma uncertainties for 14,560 galaxies, it was found that pairs of reported extra-galactic distances of the same galaxy differ from each other by 2.07 the reported uncertainties on average. In my opinion, this indicates that...
  8. ohwilleke

    I Is There Evidence to Suggest Our Universe is Anisotropic?

    The cosmological principle holds that at large enough scales, the universe is homogeneous and isotropic (i.e. symmetrical). But, there is meaningful evidence from astronomy observations of anisotropy at the largest observable scales in the universe, which a new preprint (discussed below) sets...
  9. J

    I The Hubble constant − constant in what way?

    The general form of Hubble’s law for a given cosmological time t is given by, v = H(t)D, (1) where v is the recession velocity of an object, D is its proper distance, and H(t) is the Hubble parameter at t. To get the H vs. t plot based on the ΛCDM model, we can use the following steps...
  10. k1120

    Python code to calculate the age of the Universe using the Hubble constant

    honestly just need some help here i understand the physics just not how to code it or the syntax of python
  11. T

    A Accelerated Hubble expansion -- Is the second derivative positive?

    Since distances increase, their first derivative which is velocity (Hubble constant) should be positive if not increasing too. Accelerated expansion needs the velocity to increase. What about the third derivative which is acceleration? An accelerated universe could have third derivative (called...
  12. Arman777

    I What is the role of voids in the Hubble tension?

    It has been proposed the Hubble tension can be solved if we assume our galaxy is located in a giant void (such as KBC). I am confused at this point. If we were living in a giant void, we should have measured the Hubble constant lower. Since when the light passes an underdense region it gets...
  13. Arman777

    I Understanding Void effects on the Hubble constant

    Its proposed that Voids can solve the Hubble Tension but later on with detailed studies its shown that, its not possible. And I am reading an online site and I saw a nice graph but I am troubling to understand it...
  14. phyzguy

    I New measurement of the Hubble constant is consistent with the CMB value

    This paper just came out with a new measurement of the Hubble constant based on the technique of gamma ray attenuation. The result is consistent with the lower (CMB-based) value. Interestingly, they also do a joint analysis of several non-CMB techniques (BAO+BBN+SN+γ-ray attenuation), and find...
  15. Cathr

    I On Hubble's constant, the expansion of the Universe and vacuum energy

    Recently I was thinking about the Hubble's constant (which, actually, is not Hubble's and not constant...) and wondering: if the universe is expanding at 70 km/s each Mpc, then there's possible to calculate some expansion of space, say from me to a person 1 meter away from me (theoretically)...
  16. ea251ah

    Is the Hubble Constant a Constant? Exploring the Conundrum of Varying Estimates

    Hubble constants (HCs) have been estimated based on the CMB, on Cepheid variables, gravitationally lensed quasars, Type 1 supernovae, and red giant luminosity. Not all agree within their estimated error bounds. This has been represented by some as a conundrum. Is it? The mean age of the...
  17. mfb

    I New H0LiCOW result: Hubble constant is 73.3 +1.7-1.8 km/(s*Mpc)

    The collaboration with the questionable acronym improved their measurement with a joint analysis of the whole dataset of six gravitationally lensed quasars. Measurements based on supernovae (measuring the Hubble constant "now") and measurements based on the cosmic microwave background (needing...
  18. hyksos

    I Imaging galaxies receding at a velocity that exceeds c?

    There are galaxies that are so far away that metric expansion causes them to have a co-moving recessional velocity that exceeds the speed of light. However, those galaxies are also so far away that the time it took the light to reach us was itself billions of years in the passage of its journey...
  19. mfb

    I Gaia parallaxes might reduce tension for Hubble constant

    In the last years a discrepancy between two methods to measure the Hubble constant appeared. Measurements based on redshift and the cosmic distance ladder produced results of about 73 km/(s*Mpc) while measurements based on the cosmic microwave background lead to results of about 68 km/(s*Mpc)...
  20. F

    I BAO : Relation between redshift, Hubble constant and radial

    From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : ##c\Delta z = H(z)\Delta \chi\quad\quad(1)## with ##z## redshift, ##H(z)## Hubble constant at redshift = ##z## and ##\chi## radial coordinates. One...
  21. M

    B Demystifying the Hubble Constant

    This year alone we have conflicting speeds for the Hubble Constant with 67.66 (+ or - 0.42) from the Planck Mission and 73.45 (+ or - 1.66) from the Hubble Space Telescope. The answer is simply found, and is between those figures. The furthest thing we can see (in theory) is 13.8 billion light...
  22. Ranku

    I What will happen to the Hubble value if dark energy is dynamic and decreasing?

    For constant dark energy, Hubble value will eventually become asymptotic. If dark energy were dynamic and gently decreasing, what will the value of Hubble eventually become - will it asymptote or keep decreasing?
  23. S

    A Variation of Hubble constant in cosmological time

    Is the Hubble constant decreasing over cosmological timescales?
  24. K

    I Problem Hubble parameter

    This is the time derivative to calculate the speed which a galaxy moves away from another galaxy. I don't understand how they get from da/dt (xi − x1) to (∙a)/a a(t). (xi − x1). Could anyone explain this? vi(t) = d/dt (ri(t) − r1(t)) = d/dt a(t)(xi − x1) = da/dt (xi − x1)...
  25. Q

    Calculate Comoving distance as a function of parameters

    Homework Statement I'm doing research with a Professor and I'm constructing a likelihood function which has parameters the density parameters found in the Friedman equation found using Massive Gravity Action H Homework Equations H(z)^2=a + b (1 + z) + c (1 + z)^2 + d (1 + z)^3 + r (1 + z)^4...
  26. N

    B How fast is Earth traveling due to the Hubble constant?

    I'm trying to figure out or find how fast Earth is moving due to the Hubble constant. I'm runnng into two answers and have difficulty understanding either one. --- One answer is a number of kilometers per second per megaparsec. But that's a distance per time unit per distance unit. I don't...
  27. resurgance2001

    I Variations in the local value of Hubble constant

    I have been reading in various sources that the directly measured value of Ho is high than the global value inferred by precise measurements of the CMB. The local value seems to be around 73 km/s/Mpc where as the global value inferred, assuming a flat Lamda CDM model is about 67 km/s/Mpc...
  28. G

    Particle Motion (Astrophysics)

    Homework Statement This is new for me, so forgive me my clumsiness. I am working on the following problem: A particle p is moving with a velocity v1 = c (speed of light) towards an object q, which is moving in the same direction with the speed v2, where v1>v2. Now, v2 is a function of the...
  29. M

    What is the reason we don't "see" the Universe's expansion?

    Homework Statement Ok, so if the universe is expanding, and the scale of space itself is changing, then that means that even humans and the Earth is expanding right? But is the reason why we don't notice this effect the slowness of it? So v=Hd, where H is Hubble's constant, which has a value of...
  30. resurgance2001

    I Discover the Latest Cosmological Parameters for the CDM Lambda Concordance Model

    Cosmological paramaters in the cold dark matter Lamda concordance model. I have been trying to find the most uptodate values for Ho , Ohmega m and Ohmega Lamgad I was wondering if anyone knows where I might find the most reliably, widely accepted values for these values that are currently...
  31. A

    B B Calculating the age of the universe with the Hubble constant

    Hello, When we assume the universe is expanding at a constant rate, we could say the age of the universe is 1/Ho. but I always feel unsure about that because 1/Ho is the distance over the velocity. However, the velocity changes over time, that is, when something is closer to us, the move slower...
  32. resurgance2001

    I How did scientists figure out the expansion is accelerating?

    I understand that the acceleration of the universe's expansion was discovered by looking at very far away Type 1a supernovae. My question is how was the data used exactly to calculate the Hubble constant in the past and then compare it with today's value? Did they simple plot the distances...
  33. M

    A Tension between local and CMB measurements of Hubble constant

    "A 2.4% Determination of the Local Value of the Hubble Constant" by Riess et al has led to some excited news stories recently. I don't see it discussed anywhere here. Looking for the essence of the paper, I note three things: The two measurements considered are "the Hubble constant ... measured...
  34. Jimster41

    Hubble constant and Entropy v2

    Is there agreement here regarding the relationship between expansion of the universe from its initial conditions, entropy (2nd law), time, (and more tenuously perhaps) "evolution"? I asked a question awhile back that was too specific I think that tried to tie the constant of expansion to the...
  35. Jimster41

    Hubble Constant and Entropy

    Do I understand correctly (in general terms) or wildly incorrectly if I imagine that the constant of expansion and the second law of thermodynamics are very closely connected, or even that the constant of expansion is potentially the source of the second law?
  36. gangula pranav

    Hubble constant

    if the speed of the expansion is decreasing then it would mean that the radius of the Hubble sphere is increasing because it would require more space to expand faster than the speed of light .but as you said if the speed of expansion is increasing then why is the Hubble sphere increasing and why...
  37. gangula pranav

    Exploring the Hubble Sphere & Hubble Constant

    why does the Hubble sphere increase amd why is the Hubble constant called a constant if the value keeps changing.
  38. C

    Dark Energy an effect of inhomogeneity?

    This new paper Local Large-Scale Structure and the Assumption of Homogeneity claims that a combined analysis of several surveys indicates that there is a substantial local under-density in the universe on the order of 800 MPC in size. Previous work done by other authors suggests that an...
  39. V

    Energy scale of Hubble constant for dark energy scalar field

    Hello All, In Carroll's there is a brief introduction to a dynamical dark energy in which the equation of motion for slowly rolling scalar field is discussed. Then to give an idea about the mass scale of this field it is compared to the Hubble constant, saying that it has an energy of...
  40. tomwilliam2

    Variation of Hubble constant in model universe

    Homework Statement For a problem I'm doing, I am considering a universe in which k=0, and I'm told that I can consider most of the expansion to have happened during a phase when only one of the density parameters was dominant (I know which one, as well), but I don't know the scale factor or...
  41. E

    Hubble constant from Time delays

    Hello, everyone it seems to me that people understand how to do this problem but I am struggling to solve it. Homework Statement A quasar with redshift 0.3 is gravitationally lensed into two images by an elliptical galaxy at redshift 0.18. There are two images of the quasar which are separated...
  42. L

    Hubble Constant am I gettting this right?

    Hi guys, I am in a state of slight confusion! I need to know if I am getting this information right, or I have gotten something wrong somewhere! It is to do with the Hubble constant and the Hubble law. So 1920 Edwin Hubble estimated that the Universe was 14 Billion years Old but new...
  43. P

    Redshift at 11.9 and the Hubble Constant

    Recently, astronomers from Caltech and Edinburgh University discovered galaxies with a redshift of 11.9. With the Hubble constant at 67.8 km/s/Mpc ( according to the most recent survey with the Planck Satellite ) this means that the galaxies are at a distance of 14.24 billion light years ! With...
  44. N

    Exploring the Relationship Between Hubble Constant and $\phi$

    1. Homework Statement If \phi is a usual field is it possible that H\dot{\phi}=-\partial^2\phi/{\partial x^2} Where H is the Hubble constant and the dot denotes time derivative 2. Homework Equations H\dot{\phi}=-\partial^2\phi/{\partial x^2} 3. The Attempt at a Solution I tried different...
  45. J

    Why is the age of the Universe the reciprocal of the Hubble constant?

    According to the wikipedia entry, the latest values for the Lambda-CDM model parameters for the age of the Universe, t_0, and the Hubble constant, H_0 are t_0 = 13.75 \pm 0.11 \times 10^9 \mbox{ years} H_0 = 70.4 \pm 1.3 \mbox{ km s}^{-1} \mbox{Mpc}^{-1} If you combine the errors this implies...
  46. R

    What the Hubble constant must be set to if Univ is flat

    What must the Hubble constant be if the Universe is flat. At the Lamda CDM article on wiki it says 70.4 km/s mpc. I'm not sure if that's what it must be if the universe is flat or if that's what experiments have measured it to be, I'm 99% sure that it's the former but I want to be 100% sure...
  47. J

    Hubble constant depend on expansion model?

    Does the measured value of the Hubble constant, H_0, depend on the details of the expansion model one uses?
  48. J

    Is Hubble Constant Really Constant?

    i think Hubble constant is not a constant because of relativistic speeds of galaxies. is it?
  49. D

    Alcubierre Drive vs the Hubble Constant

    Dear all, I was reading about the Alcubierre drive, and although there's the more recent issue with Hawking radiation potentially frying everything within the warp field that's generated, I was wondering whether there had been any thought given to the possibility of the drive being flung parsecs...
  50. B

    Help on Hubble Constant, H0, h100 & Critical Density Rhoc

    For the most part I've been using the Hubble constant of: H0 = 72 km/s/Mpc but I've started seeing it expressed as: H0 = 100 h100 km/s/Mpc. what is h100 and why is it coming up in this?? I've also seen the critical density for the shape of the universe (flat/open/closed ect) as: Rhoc = 1.879...