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# How fast is the universe expanding?

How fast is the universe expanding?

A galaxy cluster one megaparsec away from us is probably moving away from us at about 70 km/s. That’s Hubble’s law:

https://en.wikipedia.org/wiki/Hubble’s_law

The basic idea is that new empty space is being created at a (roughly) constant rate throughout the universe. Therefore, the further away an object is from us, the more empty space is being created between it and us, because there’s more space. And so it appears to be moving faster.

That means stuff forty-five billion lightyears from us appears to be moving away from us at the speed of light. But that’s an illusion. It’s not really moving.

Think about somebody baking raisin bread in the oven. As the bread bakes, it expands. The raisins don’t move, but they appear to get further away from each other.

That’s why it can look like things are going away from us faster than light, when they’re really not.

(The rate of creation of empty space used to be considered constant, but we believe it’s changing. We don’t know what’s causing that change.,… but we’ve given it the name dark energy. https://en.wikipedia.org/wiki/Dark_energy)

If you want a more complete description, including some about the history of Hubble and his law, you could read this article I wrote a while back.

http://www.thephysicsmill.com/2013/03/24/receding-horizons-dark-energy-and-the-expanding-universe/

Source: Jonah Miller  responds to a question in the Science on Google+ community. The best answers to questions will be posted as part of our  #Askascientist  series. Do you have science questions for us? Use the Science Outreach category to ask the science community.

## Join the Conversation

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2. The raisin bread is a lovely analogy!

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3. But if it is space/time that is expanding, then how come the dimensions change? Shouldn’t the 5cm between raisins remain 5cm, albeit a constantly stretching 5cm…?

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4. That depends on the ruler you use, Alan Kerlin . For example, a physical ruler is held together by electromagnetic force, so it doesn’t expand with the universe. So if you compare space to your ruler, the ruler says distances are getting greater.

In reality, we use the redshift of light to measure the expansion rate of the universe. As the light travels through the universe, it does indeed stretch out with the space. However, this means the frequency of the light changes with respect to the ticking of a clock on Earth. And we can measure that.

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5. Alan Kerlin  I like the way you think!  Perhaps not only our measuring instruments would stretch, but our awareness also, rendering the effect null for our actual experience of it?

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6. So the measurement stays the same, and the meaning therefore of light year actually grows? Or the light year stays the same because light can’t speed up?

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7. But we do experience the expansion, ann kiszt . This is why we observe that distant galaxies are moving away from us… the more distant, the faster they appear to be moving.

There are two possible explanations: either Earth is a very special place in the universe, or Earth is not special and the fabric of the universe itself is expanding.

The latter explanation is what is consistent with everything else we know.

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8. Alan Kerlin

the light changes. And since we are comparing the light to atoms on Earth, which are not changing, we observe the effect.

The speed of light, of course, stays the same.

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9. The wavelength of light extends in keeping with the expansion of space/time? Ok then. So C is not actually a constant then? What does that do for E=mC²?

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10. Alan Kerlin the way we actually measure the distance to distant galaxies is by their brightness. Certain stars (and supernova) have known brightnesses. So, since we know how bright the star should be, we can tell how far away it is by how dim it got.

We then compare the brightness of the stars to the redshift of their light. And this tells us both how far away from us they are, and how fast they appear to be moving away from us.

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11. Alan Kerlin no the speed of light is constant. As the wavelength expands, the frequency must drop.

Light that experiences cosmic redshift becomes, well, redder.

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12. Jonah Miller Good point, so either we must say that space is not expanding, but filling with expanding matter, or that we’re making a giant mistake on how we perceive light that has been moving for so many eons before we measure it.

We say it’s constant, yet we perceive differences in things that relatively remain in a fixed relationship.  If their expansion is a fixed relationship down to the last particle of matter then we could not possibly measure it as our measuring instruments would expand in relation also.

Wouldn’t it blow everyone’s minds if we discovered that the speed of light is NOT constant but is affected in it’s transmission in some way?

I apologize if I”m way off base.  I am not a physicist, just a philosopher.

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13. ann kiszt I’m not sure I follow you. The choice was between space expanding or Earth being special.

I’m (and most physicists are) pretty sure that the speed of light is constant. But that doesn’t mean physicists aren’t carefully checking whether or not that’s true!

There’s always a precision experiment or two trying to test whether or not the speed of light is constant. So far, the consensus is that it is indeed constant.

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14. Gaah! Does my head in this stuff… Light speed stays the same, but our ruler hasn’t stretched. So from light’s perspective, our ruler is shrinking, but light still does 3×10^8 m/s?

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15. Alan Kerlin , yes exactly. From the perspective of a ray of light, our ruler shrinks. But the speed of the light is always 3×10^8 m/s. As the light stretches, the frequency of the light drops just so to keep the speed constant.

And you can think of the drop in frequency of the light as related to how it sees our ruler shrinking.

Think of it this way. Say on Earth I create a beam of light that I know should be identical to the one emitted by a distant galaxy. I then compare my beam of light to the beam of light that came from the distant galaxy. My light beam will be bluer in color than the one that travelled through space, because the one that travelled through space got stretched out by the expansion of the universe.

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16. I make a throwaway jest and score a goal. 😁

I start to understand how a religion believer must feel. “It offends all sense of logic, but you need to just accept it and move on.”

Thanks for sticking with me on this.

A related issue, if I may. If you are too busy with exams and all, I understand.

I’ve often wondered about what’s the go with space/time from a photon’s perspective? No mass cos Einstein. So light can go at the speed of light. Durr… But that means time is foreshortened infinitely. So for the photon, time ceases to exist but for us the photon is immortal?

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17. I hope I helped, Alan Kerlin . I don’t want relativity to feel like religion!

In answer to your question regarding how time passes for a photon.

Right, so photons, even in special relativity, don’t experience time. They travel along so-called “null paths.”

So from a photon’s perspective, it’s birth and death, and everything in between happen at the same “moment.”

A photon can be absorbed or emitted, but it will never decay into anything else. In that sense, it is immortal.

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18. Alan Kerlin

if you’re familiar with the electromagnetic description of light, it is perhaps not surprising that photons don’t experience time.

Light is made of a changing electric field driving a changing magnetic field, which drives a changing electric field, ad infinitum.

The speed of light is the rate that these fields drive each other.

But this means that if you travel along a light wave at the speed of light, you wouldn’t see any change at all! You’d see a static electric field and a static magnetic field!

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19. Hence why I try to bend my head around it. Because it isn’t a religion…

So a photon can get away with not having any mass, even though it behaves kind of like it does have mass in some respects (waves etc), because it only actually exists for an infinitely small moment of time (from it’s own perspective)?

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20. Jonah Miller but isn’t the data also consistent with light simply drifting into lower and lower frequencies over time, and the distant objects being stationary? Wouldn’t that also look like red-shift? Have we ever seen anything very distant that is not red shifted? How would we tell those two cases apart?

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21. Alan Kerlin  Good to hear. 🙂

I’m not sure “it can get away” with having no mass. Masslessness is simply a property that some particles have. They don’t need to get away with it.

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22. Well I am thoroughly enjoying the learning provided here, even though a good part of it is too unfamiliar to stick!  Thanks guys! I really got my brain tickled today and I love that!

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23. You’re referring to the tired light hypothesis, Marc Mengel . It’s a good idea. Lots of famous scientists have proposed it.

Unfortunately, the tired light hypothesis doesn’t match all the evidence. For one thing, we’re now capable of measuring the brightness of the “surfaces” of distant galaxies precisely. In a tired light universe, the surface brightness of a distant galaxy should be proportional to 1/distance^2. It should follow an inverse square law.

However, we’ve observed that brightness actually falls off as 1/distance^4. This is because galaxies appear closer than they actually are, because the light was emitted before they were “expanded” away.

This is the particular experiment:

https://en.wikipedia.org/wiki/Tolman_surface_brightness_test

And of course, a tired light hypothesis is inconsistent with our observation of the cosmic microwave background.

And here’s some more info, describing more sophisticated tired light hypotheses, and how they are inconsistent with observation:

https://en.wikipedia.org/wiki/Tired_light

https://medium.com/starts-with-a-bang/how-the-big-bangs-alternatives-died-aa52857cf943#.2gftnbgqf

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24. Glad to hear it, ann kiszt ! 🙂

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25. “You shouldn’t anthropomorphise XXX – they really don’t like it.”

Yeah I was just shorthanding. I was wondering if there is a connection between speed of light, no mass and interacting in wave/particle-like manner.

Have really appreciated you sticking it out on this thread.

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26. I’m happy to hang around and explain my favorite topic, Alan Kerlin . 🙂

I see what you’re asking. Mass and speed are obviously related. Massless particles always go the speed of light and massive particles accelerate. What’s deep there is that this is actually how we define mass. I wrote about this a while back:

As for interacting in a wave/particle-like manner… Everything is a particle and a wave, massless or massive… so I don’t think that’s particularly related. The de Broglie relation

lambda = h/p

is actually always true. You don’t need mass for that because momentum can be defined for massless particles.

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27. Ah… When I went to school, the dual personality of light was something of a conundrum. At least that I recall….

So nothing massless has zero velocity? That’s interesting. I wonder why.

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28. Alan Kerlin that’s right, massless particles all always go the speed of light.

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29. Jonah Miller Therefore, the frequency of ‘Light’ will eventually equal zero. Right?

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30. why the scare quotes around the word “light,” David Whitmore ?

In the “infinite future,” the frequency of light from the cosmic microwave background will vanish, yes.

Or are you referring to the redshift of things far away from us, which will eventually appear to be moving away from us at the speed of light? Yes, you can think of that “appearance of motion” as coming from a redshift that brings the frequency of the light to zero.

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31. Jonah Miller

You said:

“I’m (and most physicists are) pretty sure that the speed of light is constant. But that doesn’t mean physicists aren’t carefully checking whether or not that’s true!”

Has the Constantness of the Speed of Light really and truly been proven? Or is it just that we haven’t been able ‘break’ it (disprove the theory) yet?

We (you physicist-types) know that all sorts of materials can and do change the direction of a Light wave; at least, that’s what we non-scientist types get taught in schools. Am I right?

If gravity waves form deflecting lenses, and there might be multiple lens from the point of origin to where we can ‘see’ the light; how can we truly know how far away the light source is, and how far it really travelled to get here? Not to mention, where the source really was when it broadcast said light towards us?

Another question I have is “Do we see the ‘light wave’ or the ‘light particle’ (I’m assuming here that the particle is a photon). But then, I’ve also been taught that ‘Light’ is comprised of two parts; a ‘wave’ and a ‘particle’. Because according to various theories I’ve heard about, for a physical object (the photon) to travel at the speed of light, it has to have infinite mass. Right? Does the ‘gravity lens’ affect both parts of light?

“There’s always a precision Experiment or two trying to test whether of not the speed of light is constant. So far, the consensus is that it is indeed constant.”

Since when is ‘consensus’ considered proper scientific procedure?

It was consensus that claimed things like ‘the earth was flat’ and the ‘Sun orbited the Earth’, or ‘the Earth is the Centre of the Universe, because everything in the Universe is moving away from us’ (except, of course, the Andromeda galaxy, which is on a collision course with the Milky Way Galaxy).

Is the Earth’s orbit increasing?

I have so many questions, I just don’t know where or what is the proper way to ask whom.

Thanks for what you have already taught me.

Ever a student.

David.

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32. That’s the “Scientific Method” David Whitmore​.

And its why common usage of the word theory is wrong.

“It’s just a theory… ”

What people actually mean is hypothesis.

A hypothesis is an assumption, an idea to be tested. A theory is a hypothesis that has been repeatedly tested and shown to be correct. People may continue to run experiments to try to break that theory from different angles, but if it continues to be proven, then there comes a time when you have to sccept it as a Law. The Theory of Evolution and Survival of the Fittest is a good example. So is global warming.

Funnily though, the Law of Gravity is now being found wanting. It has worked perfectly on the normal human scale, but breaks down at the quantum scale. That’s at the core of what many physicists are trying to get to the bottom of these days.

The way Jonah has framed his responses is normal practice for scientists, to express what the rest of us think of as a certainty, instead with a degree of uncertainty. Because even a bedrock like the Law of Gravity can be found incomplete once we learn more about the context in which it operates.

That doesn’t make it wrong. Or scientists wrong. It’s just like the scientist who means something very different when they say “theory”.

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33. Alan Kerlin – Thank you Sir.

I have found it more and more difficult to translate various mathematical and scientific jargon into what I know as ‘everyday language’. Especially as I attempt to learn more about things that I find interesting.

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34. Jonah Miller – When I was taught about the electromagnetic spectrum (many decades ago), I learned that light was only one particular segment of the spectrum, and I wanted to differentiate light from Gamma waves, sound waves, X-Rays, Radio waves, and all the other parts of that Spectrum chart.

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35. David Whitmore , sorry for the confusion. Yes, what Alan Kerlin says is correct.

I can clarify a bit though.

Has the Constantness of the Speed of Light really and truly been proven? Or is it just that we haven’t been able ‘break’ it (disprove the theory) yet?

For all intents and purposes, the constant-ness of the speed of light in vacuum is experimental fact, tested to exquisite precision. But science is about questioning what you believe to be true and some people hypothesize that on the smallest length scales, the speed of light may not be constant after all. But let me be perfectly clear. If the speed of light is not constant after all, it will only be noticeable in situations so extreme that we would never encounter them even in the LHC at CERN. We’re talking the cores of black holes or in the very very early universe, far earlier than we can currently probe.

We (you physicist-types) know that all sorts of materials can and do change the direction of a Light wave; at least, that’s what we non-scientist types get taught in schools. Am I right?

Yes that’s right. And the speed of light in materials can change. It’s only constant in vacuum. But the “vacuum” number is what limits the speed information can travel in any situation ever.

If gravity waves form deflecting lenses, and there might be multiple lens from the point of origin to where we can ‘see’ the light; how can we truly know how far away the light source is, and how far it really travelled to get here? Not to mention, where the source really was when it broadcast said light towards us?

Excellent question! If we relied solely on one observation, the light from (say) a supernova, we could not! We have to synthesize multiple observations. When we observe the light from a distant supernova, we also observe the galaxy it is in. And the galaxies around it. And we use our mathematical models of how gravity and gravitational waves should behave to do statistical analysis.

Long gone are the days of astronomy when you could look into the the sky and write down what you saw. Nowadays you must take a photograph and do sophisticated analysis on it.

Another question I have is “Do we see the ‘light wave’ or the ‘light particle’ (I’m assuming here that the particle is a photon).

Both! They are one and the same.

You say:

I’ve also been taught that ‘Light’ is comprised of two parts; a ‘wave’ and a ‘particle’.

Not quite. Light isn’t composed of two parts. The wave and the particle are two ways of looking at the same thing. Sometimes light acts like a particle. And sometimes it acts like a wave. But really, it’s something else, which we have no intuition for because we don’t encounter it in our daily lives. That’s the particle-wave duality of quantum mechanics. If you’re interested, I did a series on this a while back:

http://www.thephysicsmill.com/2012/12/09/the-charming-doubleness-particle-wave-duality/

http://www.thephysicsmill.com/2012/12/24/unreal-truths-the-bohr-model-of-the-atom/

My preferred way of thinking about it is this. Everything is a wave, but waves can be made to act like particles. I wrote about that too:

http://www.thephysicsmill.com/2015/01/18/whats-particle/

http://www.thephysicsmill.com/2015/01/25/sometimes-particle-isnt-possible/

tl;dr: Gravity effects light, which is both a particle and a wave.

You say:

Because according to various theories I’ve heard about, for a physical object (the photon) to travel at the speed of light, it has to have infinite mass. Right?

I’m not sure where you heard that, but it’s super wrong. Photons are massless particles. They have zero mass, which is why they can travel the speed of light. Massive particles cannot go the speed of light because they cannot accelerate to the speed of light. It’s actually pretty easy to see this if you start from the assumption that the speed of light is constant. I work through the thought experiment here:

http://www.thephysicsmill.com/2012/11/25/the-universal-speed-limit/

Since when is ‘consensus’ considered proper scientific procedure?

When scientists say “concensus,” we mean that there’s overwhelming evidence and that, for all intents and purposes it’s a fact. But as scientists, it’s our job to question our assumptions. We’ve been wrong before, after all. That’s what Alan Kerlin was saying.

Is the Earth’s orbit increasing?

The expansion of the universe doesn’t effect the Earth’s orbit because the sun’s gravity beats the expansion. (And that’s because it’s sufficiently close to us.)

Now, in principle, the Earth is radiating gravitational energy as it rotates around the sun. So in principle, the orbit isn’t permanent and the Earth would eventually fall into the sun. But this effect is so tiny that it will essentially never happen in the age of the universe.

And if you ignore general relativity, the Earth is more likely to fly away from the sun than fall into it. That’s what’s happening to the moon because of the way the moon and the sun distort each other.

That’s not really happening to the Earth, though. The sun is too big and too far away. But When the sun goes red giant on us, it will get much less dense as it expands. When that happens, the Earth could be 0.5 AU further away from the center of the sun than it is now. Unfortunately, that will still be INSIDE the sun when it goes red giant, since it will expand significantly.

I hope that helps you.

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36. David Whitmore regarding “light” versus other parts of the electromagnetic spectrum:

I tend to call everything in the electromagnetic spectrum “light” and call the stuff we can see “visible light.”

You need to use different telescopes to “see” each wavelength, but we’re getting pretty good at that.

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37. I do enjoy the conversation of the proton having no mass.

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38. Terry Coleman wait what? The proton definitely has mass. Do you mean the photon?

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39. Jonah Miller – Thank you for spending so much time with me. I will be checking out the articles you referenced. I’m still learning and relearning.

My latest ‘formal’ science education is more than 35 years old; so it may be a bit out-dated (and probably is).

Since my forced retirement, I’ve been trying to educate myself. Space, and space travel has always intrigued me, and I can’t wait to get out there.

Have a great night.

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40. I’m glad I could help, David Whitmore ! 🙂

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