# What is Hubble's law: Definition and 40 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. ### B Which statement is more accurate? (Hubble's redshift vs. Hubble's law)

A. The two great cosmological discoveries of this century, Hubble's redshift and the cosmic microwave background, have made the Big Bang theory the most credible theory of the origin of the universe so far. B. The two great cosmological discoveries of this century, Hubble's law and the cosmic...
2. ### Cosmology: Hubble's law and Redshift

I tried to attempt this by taking z=dv/c but I saw that relation Liddle's book on Cosmology and I really don't know where this came from. Secondly, I will need some series ( maybe Bionomial) in terms of z from which I can show small z. I think it's simple but here it's not clicking to me.
3. ### Hubble's Law and Doppler Shift

(a) How far would a galaxy be whose speed with respect to the Earth is ##c##? Would it be observable from the Earth? r=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyearsr=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyearsr=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyears...
4. ### I Challenges to the Redshift Method and Hubble's Law

Since Hubble's Law has been around a long time, so, after almost 100 years, to challenge its validity looks like dumb and stupid. So let me be that dumb guy, or maybe the bad boy that spoils the whole thing. Here let me start with the Redshift in general. There are three Redshifts proposed till...
5. ### I Hubble's Law and Star Velocity

Hubble's Law and Star Velocity By using Hubble's law and the principle of velocity superposition of relativity, we can deduce when the distance of star from the observation point is (photometric distance)r, its velocity V can be expressed as: V = Csin (Hr/C) Therefore, the...
6. ### I Hubble's law and conservation of energy

2 bodies that have distance d between them are distancing from each other because Hubbles law. at time t=0 distance between them was d(0) and speed between them was 0. If no force interacts with them then distance is increasing by rate ##\frac{\partial d}{\partial t}=H_0*d## Is it correct...
7. ### I Hubble's Law: Distance, Speed, & Upper Limit

I have read that Hubbles law states that a galaxys speed as it moves away from us is proportional to it's distance. Does this have an upper limit? If a galaxy 1 megaparsec is moving at 70 km/s, a galaxy 10,000,000 mgp away is moving faster then light. Or am I (most likely) mistaken?
8. ### B Does Hubble's Law imply that galaxies are accelerating away

Is I understand it Hubble's law states that V = Hd = dx/dt Solving this differential equation, I got d = kexp(HT) where k is an arbitrary constant. This implies d²x/dt² = Hv = H²d and dⁿx/dtⁿ = Hⁿd. However (at least for me), finding the value of k is a problem, for 1, it must vary from galaxy...
9. ### I New possible explanation for Hubble redshift

I just saw this news article on the home page of the university I'm working in: http://www.aalto.fi/en/current/news/2017-06-30/ It seems to be about a study that can potentially explain the redshift of distant stars with a mechanism that's related to the interaction of photons with the...
10. ### I Hubble's Law, Friedman Models & Spacetime Curvature Explained

Is the Hubble's law(recessional velocity linearly proportional to distance) valid for all cases even when the spacetime is curved? Is there a nonlinear model for Friedman models or it's always linearly proportional?
11. ### B Hubble's Law: Explaining Why Distant Bodies Recede Faster

I am aware of the philosophical annotations with this question as some argue that science should only serve to explain how physical systems occur and not why; however if this question is not confined to this then I hope that there are answers. Is there an explanation to why Hubble's law exist...
12. ### A What distance is used in Hubble's Law.

Hello, I was thinking about the Hubble's law and I know that it's determined as H(t)=\frac{1}{a(t)}\frac{da(t)}{dt} and then, thinking in the derivate of the scale factor as the speed, we've de usual formula v(t)=H(t)D But my question is: the distance is the distance we observe the object (so...
13. ### Hubble's Law of Redshifts using observed distances

The problem statement, all variables, and given/known data: Assume that the radial velocities vr of galaxies, at the present time, are given by Vr=H0*r, where H0 = 65 km/(s*Mpc). However, we do not observe the present distances of galaxies, but the distances they had when light left them...
14. ### Calculating Galaxy Distance for Light Speed using Hubble's Law

The recessional velocity of a galaxy can be calculated using Hubble's Law; the equation where Hubble's constant multiplied by the distance to the galaxy equals the recessional speed of that galaxy. Thus, what is the distance to a galaxy that is required so that the recessional velocity of that...
15. ### Relativistic Doppler and Hubble's law

I have a quick question about the Special Relativity. Non-relativistically, we expect no doppler shift in the wavelength of the emitted light if the source is moving at right angles to the line of sight to the observer. However there is a transverse doppler shift even in this case, caused by...
16. ### Hubble's Law and Hubble's Constant

Hi there! I'm having some trouble understanding Hubble's Law and Constant and would be grateful for some help. So Hubble's Law shows that the most distant galaxies have the greatest red-shift, as they are moving away from us fastest. This shows that all galaxies are moving away from each other...
17. ### Earth's velocity wrt CMB gives the centre of the universe?

Alan Guth mentioned in his lectures that to get the accurate linear relationship of Hubble's law, we need to subtract the velocity of the Earth with respect to the cosmic microwave background (CMB) when calculating the velocity of galaxies. Question 1: If space is expanding uniformly...
18. ### Dark energy and type 1a supernovae?

I am trying to wrap my brain around the evidence for accelerating expansion of the universe from type 1a supernovae. From what I understand, it was first realized that the universe was expanding at an increasing rate from discrepancies between the calculated distances to type 1a supernovae using...
19. ### CMB dipole anisotropies & Hubble's Law

It is known that the dipole anisotropies of the CMB radiation, can give the relative velocity of the Earth with respect to the CMB "rest" frame... In first order, this is given by \beta \approx 1.2 \times 10^{-3} or u_{CMB} \approx 360 km/s . I have one question here: Why is the velocity given...
20. ### Velocity of receding galaxies. Hubble <-> scale factor

Hi there, This is my first post but I've been a spectator for a long time now. So I've been working on some of the basics of cosmic expansion and there is one contradiction that I came upon that I can't seem to resolve. I've looked around some of the similar threads but I couldn't find anything...
21. ### Questions on Hubble's law and Friedmann equation

The Hubble's Law v=H0D LHS of The Friedmann equation H2/H02 I am a bit confused with the following: 1: The Hubble constant H0 is the current universe velocity/distance ratio, it is basically a constant, so galaxies in the universe with a further distance, move away from each other at a...
22. ### Solve Hubble's Law: Proxima Centauri & Sun in Milky Way

Homework Statement Determine for the nearest star what should be it's recessional velocity if the space between the two stars is expanding according to the Hubble's law. Compare this value with the speed of the sun when it moves with Milk Way. Homework Equations H_{0} = \frac {v}{D}...
23. ### Hubble's law and the age of the universe

Hubble's law states that the speed of a distant object is proportional to it's distance (this implies an expanding universe). By finding the distance and speed (via redshift) of various distant galaxies you can produce a graph displaying the linear relationship. The gradient of the line is...
24. ### Hubble's law and the age of the universe

Just did a test today (so no results back yet) (year 12 physics) and it annoyed the heck out of me, in part because to answer these questions you had to make preposterous assumptions, assumptions which in hindsight I probably should have just stated as opposed to writing out explanations as to...
25. ### Questioning Hubble's Law: Does the Expanding Universe Theory Hold Up?

The Hubble's Law is based on empirical evidence that those objects further from us are moving away from us proportionate to their distance from us. My question is that the further we look back the further we look back in time. And so, why is that when we see further, and notice objects moving...
26. ### Hubble's Law and it's relation to Quantum Mechanics

Can someone say a few words on Hubble's law as (if) it relates to QM?
27. ### Hubble's law and cosmological density

While reading I came across this chart in Wikipedia http://en.wikipedia.org/wiki/Metric_expansion_of_space Anybody know the source or a similar plot with some explanations? What do you make of it?? What does it purport to show? Is it in the correct section [Theoretical basis and first...
28. ### Exploring Hubble's Law Conundrum with Two Planets and a Star

Imagine this;- Two planets and a star separated by cosmological distances that share an amost linear positional relationship - the star is not in the middle and both planets have a direct eyeline to it. Both planets have intelligent life and both are observing the star's redshift. Two...
29. ### Hubble's Law And A Question Of Time

One thing that really bothers me about Hubble's law is the question of time. Was this ever taken into account? When we observe large distances in space we are also looking way back in time. Sure Galaxies appear to be moving faster at a greater distance but how do we know how fast they are moving...
30. ### Is gravitational redshift taken into account in Hubble's Law?

From what I understand: 1. The more massive the object, the stronger the gravitational field. This leads to the light being emitted from the surface to shift down in frequency. 2. The rate of expansion of the universe causes a redshift proportional its distance away. I'm new to this, but...
31. ### Universe expansion and hubble's law

I've been reading about the red-shift, the universe expanding, ect... And about the observation that the further a galaxy is from us, the faster it is moving away from us, accounting for an ever expanding universe. I'm also curious about the fact that the further the galaxy, the further 'in...
32. ### Does Hubble's law implies accelerated expansion?

I`ve read that the universes expansion is accelerating. To explain this we have introduced concept of Dark matter and energy. However, Hubble's law: v = H*d means that a speed that separates two points is proportional to their distance. But since they are moving apart the distance becomes...
33. ### Hubble's Law and Voids in the Universe: Exploring the Impact on Galaxies

Would the greatest voids in the universe support Hubble's law to a substantially lesser radius than the neighborhoods of galaxies?
34. ### Does Hubble's law rely on the Doppler shift?

If not Doppler, what type of transformation does Hubble's law utilize?
35. ### Hubble's Law: Can somone be nice and answer this.

How are the distance and speed of a galaxy related?
36. ### Hubble's law and expansion question

Hi, I was looking through past exams for one of my courses, and I came across a question I wasn't quite sure how to do, so I was wondering if anyone could give some hints (or possibly the rationale for solution), thanks! Homework Statement Observers in three universes A, B, and C measure...
37. ### Hubble's Law: Exploring Beyond the Speed of Light

Since the galaxies are receeding with velocity propotional to distance, I'm curious what happens at or beyond the point where the velocities approach the speed of light & how that's possible. One source I looked at said the galaxies are invisible. Another source said the spacetime is what's...
38. ### Exploring an Alternative to Hubble's Law

I was reading one of John Gribbin's (popular science)books a few days back when I thought if there could be alternative explanation(s) for the Hubble's law.One simple explanation could be that the value of G(universal gravitational constant) could be changing with time--if G were higher in the...
39. ### Hubble's law and redshift

How would one use hubble's law to find the redshift of a galaxy 10^10 light years away? (in other words 3*10^9 pc) All I know however is the version of hubble's where v=H_o d, with H_o=71 \frac{m}{s*pc}, which tells me that v= 2.2*10^11 m/s. But this violates the second principle of...
40. ### Hubble's law- little bit of help

Hey, I am really interested in cosmology especially the acceerating vs. decelerating universe debate and I was wondering if anyone could help me out with what the implications hubble's law acutually are. I know that is states that somthing that is twice as far away then it is moving twice as...