How can space travel faster than the speed of light?

February 23, 2015 by Vanessa Janek, Universe Today
Light speed is often spoken of as a cosmic speed limit… but not everything plays by these rules. In fact, space itself can expand faster than a photon could ever hope to travel.

Cosmologists are intellectual time travelers. Looking back over billions of years, these scientists are able to trace the evolution of our Universe in astonishing detail. 13.8 billion years ago, the Big Bang occurred. Fractions of a second later, the fledgling Universe expanded exponentially during an incredibly brief period of time called inflation. Over the ensuing eons, our cosmos has grown to such an enormous size that we can no longer see the other side of it.

But how can this be? If 's velocity marks a cosmic speed limit, how can there possibly be regions of spacetime whose photons are forever out of our reach? And even if there are, how do we know that they exist at all?

The Expanding Universe

Like everything else in physics, our Universe strives to exist in the lowest possible energy state possible. But around 10-36 seconds after the Big Bang, inflationary cosmologists believe that the cosmos found itself resting instead at a "false vacuum energy" – a low-point that wasn't really a low-point. Seeking the true nadir of vacuum energy, over a minute fraction of a moment, the Universe is thought to have ballooned by a factor of 1050.

Since that time, our Universe has continued to expand, but at a much slower pace. We see evidence of this expansion in the light from distant objects. As photons emitted by a star or galaxy propagate across the Universe, the stretching of space causes them to lose energy. Once the photons reach us, their wavelengths have been redshifted in accordance with the distance they have traveled.

This is why cosmologists speak of redshift as a function of distance in both space and time. The light from these distant objects has been traveling for so long that, when we finally see it, we are seeing the objects as they were billions of years ago.

The Hubble Volume

Redshifted light allows us to see objects like galaxies as they existed in the distant past; but we cannot see all events that occurred in our Universe during its history. Because our cosmos is expanding, the light from some objects is simply too far away for us ever to see.

The physics of that boundary rely, in part, on a chunk of surrounding spacetime called the Hubble volume. Here on Earth, we define the Hubble volume by measuring something called the Hubble parameter (H0), a value that relates the apparent recession speed of distant objects to their redshift. It was first calculated in 1929, when Edwin Hubble discovered that faraway galaxies appeared to be moving away from us at a rate that was proportional to the redshift of their light.

Dividing the speed of light by H0, we get the Hubble volume. This spherical bubble encloses a region where all objects move away from a central observer at speeds less than the speed of light. Correspondingly, all objects outside of the Hubble volume move away from the center faster than the speed of light.

Yes, "faster than the speed of light." How is this possible?

The Magic of Relativity

Two sources of redshift: Doppler and cosmological expansion; modeled after Koupelis & Kuhn. Bottom: Detectors catch the light that is emitted by a central star. This light is stretched, or redshifted, as space expands in between. Credit: Brews Ohare

The answer has to do with the difference between and general relativity. Special relativity requires what is called an "inertial reference frame" – more simply, a backdrop. According to this theory, the speed of light is the same when compared in all inertial reference frames. Whether an observer is sitting still on a park bench on planet Earth or zooming past Neptune in a futuristic high-velocity rocketship, the speed of light is always the same. A photon always travels away from the observer at 300,000,000 meters per second, and he or she will never catch up.

General relativity, however, describes the fabric of spacetime itself. In this theory, there is no inertial reference frame. Spacetime is not expanding with respect to anything outside of itself, so the the speed of light as a limit on its velocity doesn't apply. Yes, galaxies outside of our Hubble sphere are receding from us faster than the speed of light. But the galaxies themselves aren't breaking any cosmic speed limits. To an observer within one of those galaxies, nothing violates special relativity at all. It is the space in between us and those galaxies that is rapidly proliferating and stretching exponentially.

The Observable Universe

Now for the next bombshell: The Hubble volume is not the same thing as the observable Universe.

To understand this, consider that as the Universe gets older, distant light has more time to reach our detectors here on Earth. We can see objects that have accelerated beyond our current Hubble volume because the light we see today was emitted when they were within it.

Strictly speaking, our observable Universe coincides with something called the particle horizon. The particle horizon marks the distance to the farthest light that we can possibly see at this moment in time – photons that have had enough time to either remain within, or catch up to, our gently expanding Hubble sphere.

And just what is this distance? A little more than 46 billion light years in every direction – giving our observable Universe a diameter of approximately 93 billion light years, or more than 500 billion trillion miles.

(A quick note: the particle horizon is not the same thing as the cosmological event horizon. The particle horizon encompasses all the events in the past that we can currently see. The cosmological event horizon, on the other hand, defines a distance within which a future observer will be able to see the then-ancient light our little corner of spacetime is emitting today.

In other words, the particle horizon deals with the distance to past objects whose ancient light that we can see today; the cosmological event horizon deals with the distance that our present-day light that will be able to travel as faraway regions of the Universe accelerate away from us.)

Fit of redshift velocities to Hubble’s law. Credit: Brews Ohare

Dark Energy

Thanks to the expansion of the Universe, there are regions of the cosmos that we will never see, even if we could wait an infinite amount of time for their light to reach us. But what about those areas just beyond the reaches of our present-day Hubble volume? If that sphere is also expanding, will we ever be able to see those boundary objects?

This depends on which region is expanding faster – the Hubble volume or the parts of the Universe just outside of it. And the answer to that question depends on two things: 1) whether H0 is increasing or decreasing, and 2) whether the Universe is accelerating or decelerating. These two rates are intimately related, but they are not the same.

In fact, cosmologists believe that we are actually living at a time when H0 is decreasing; but because of dark energy, the velocity of the Universe's expansion is increasing.

That may sound counterintuitive, but as long as H0 decreases at a slower rate than that at which the Universe's expansion velocity is increasing, the overall movement of galaxies away from us still occurs at an accelerated pace. And at this moment in time, cosmologists believe that the Universe's expansion will outpace the more modest growth of the Hubble volume.

The observable universe, more technically known as the particle horizon.

So even though our Hubble volume is expanding, the influence of appears to provide a hard limit to the ever-increasing observable Universe.

Our Earthly Limitations

Cosmologists seem to have a good handle on deep questions like what our observable Universe will someday look like and how the expansion of the cosmos will change. But ultimately, scientists can only theorize the answers to questions about the future based on their present-day understanding of the Universe. Cosmological timescales are so unimaginably long that it is impossible to say much of anything concrete about how the Universe will behave in the future. Today's models fit the current data remarkably well, but the truth is that none of us will live long enough to see whether the predictions truly match all of the outcomes.

Disappointing? Sure. But totally worth the effort to help our puny brains consider such mind-bloggling science – a reality that, as usual, is just plain stranger than fiction.

Explore further: How fast is the universe expanding?

Related Stories

How fast is the universe expanding?

February 10, 2015

The Universe is expanding, but how quickly is it expanding? How far away is everything getting from everything else? And how do we know any of this anyway?

Image: Smile, and the universe smiles with you

February 11, 2015

An upbeat-looking galaxy cluster appears to smile at us in a newly released image from the NASA/ESA Hubble Space Telescope. The cluster - designated as SDSS J1038+4849 - appears to have two eyes and a nose as part of a happy ...

Is the universe actually shrinking?

February 3, 2015

Whoa, here's something to think about. Maybe the Universe isn't expanding at all. Maybe everything is actually just shrinking, so it looks like it's expanding. Turns out, scientists have thought of this.

Study finds possible alternative explanation for dark energy

December 30, 2014

(Phys.org)—Dark energy is an unknown form of energy that is proposed to drive the accelerated expansion of the universe. A new study by University of Georgia professor Edward Kipreos suggests that changes in how people ...

Why is Andromeda coming toward us?

January 23, 2015

I don't want to alarm you, but there's a massive galaxy heading our way and will collide with us in a few billion years. But aren't most galaxies speeding away? Why is Andromeda on a collision course with the Milky Way?

What's happening in the universe right now?

January 30, 2015

There are some topics that get a little frustrating in their pedantry, but can really draw attention to the grand scope and mechanics in our Universe. This is definitely one of them.

Recommended for you

New research challenges existing models of black holes

January 19, 2018

Chris Packham, associate professor of physics and astronomy at The University of Texas at San Antonio (UTSA), has collaborated on a new study that expands the scientific community's understanding of black holes in our galaxy ...

Neutron-star merger yields new puzzle for astrophysicists

January 18, 2018

The afterglow from the distant neutron-star merger detected last August has continued to brighten - much to the surprise of astrophysicists studying the aftermath of the massive collision that took place about 138 million ...

New technique for finding life on Mars

January 18, 2018

Researchers demonstrate for the first time the potential of existing technology to directly detect and characterize life on Mars and other planets. The study, published in Frontiers in Microbiology, used miniaturized scientific ...

33 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Doug_Huffman
5 / 5 (5) Feb 23, 2015
Trite click-bait. Space is not constrained to the speed of light, but only energy is so constrained.
Gigel
2 / 5 (3) Feb 23, 2015
I wonder what happened to incoming light if a region of space in between started to move faster than light. Maybe we could detect such events.
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
tadchem
4 / 5 (4) Feb 23, 2015
The speed of light applies to anything posessing electromagnetic energy.
Space has no electromagnetic energy of it's own, nor any inertia. Lorentz' Laws do not apply.
maxwell_bean
2 / 5 (1) Feb 23, 2015
I'm confused I thought the redshift was from the velocity, not from a loss of energy as the universe expands. If something is close by but moving rapidly away does it have the same redshift as something far away but moving away at the same speed?

Also, how can we see all the way back to the time of the big bang (the background radiation) and yet still have objects further away then that?
movementiseternal
Feb 23, 2015
This comment has been removed by a moderator.
Dethe
2 / 5 (4) Feb 23, 2015
The space itself cannot move by the whole definition of space: if we would observe something in motion, it will be only obstacle INSIDE of space, not the space itself. Even at the water surface the water surface remains always invisible by its waves - only density fluctuations, solitons, bubbles or floaters or another obstacles can move or propagate along it.
baudrunner
2.3 / 5 (3) Feb 23, 2015
Explanations of red shift continue to elude the age factor of the photonic wave being observed. The oscillations of photonic wavelengths decay over time and distance.

In seawater, a 10 KHz sound wave is attenuated to 1% of its value over a distance of 38 km. A 10 KHz radio wave over a distance of only 5.8 meters. Should photonic wavelengths behave differently?

I think not. Remember, in physics, the terms "particle" and "object" are interchangeable.
jburchel
4.2 / 5 (5) Feb 23, 2015
"lowest possible energy state possible", redundant. Somebody should proofread these articles prior to posting. Also, presumably 1050 means 10^50, but should either be written accordingly or even better, notated correctly with the raised notation most familiar to modern readers...
WillieWard
2 / 5 (6) Feb 23, 2015
Space travel can be faster than the speed of light by employing phased electrodynamic waves to warp spacetime. http://youtu.be/6zh9abFF3ZE
arom
Feb 23, 2015
This comment has been removed by a moderator.
Shootist
4.7 / 5 (3) Feb 23, 2015
"lowest possible energy state possible", redundant. Somebody should proofread these articles prior to posting. Also, presumably 1050 means 10^50, but should either be written accordingly or even better, notated correctly with the raised notation most familiar to modern readers...


picking nits is such fun
ROBTHEGOB
1.7 / 5 (11) Feb 24, 2015
There was no Big Bang. Get over it.
jsdarkdestruction
5 / 5 (4) Feb 24, 2015
Movementiseternal, did you really need 6 posts essentially repeating the same
Thing over and over?do you think if you repeat yourself enough people will believe you or
Take you seriously?

I save this next part respectfully and mean no offense but instead of
Spamming the same thing over and over again here you should work on your english skills.
Bob Osaka
1.3 / 5 (3) Feb 24, 2015
Has anything outside the Hubble volume been observed moving faster than light?
Don't bother answering rhetorical questions.
There is a problem with the description of General relativity, it is a geometrical description of space time. The fabric has structure, it is therefore "something" not "nothing." There is no empty space and purest vacuum possible is filled with waves and particles, some just popping in and out of existence.
The description of the speed of light is also imprecise, three hundred million meters per second is rounded up, exceeding the speed of light, over astronomical distances such errors of laziness corrupt results.
Dark energy is theorized as some unknown particle decay which increases, decreases and then about 5 billion years ago started increasing again. Dark energy may not be infinite.
This article is Inflation theory salesmanship. Well written for puny minds to wrap their heads around but the product is missing parts, so not buying it yet.
Mimath224
not rated yet Feb 24, 2015
@Doug_Huffman copy that! Shadow is an example.
movementiseternal
Feb 24, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 24, 2015
This comment has been removed by a moderator.
movementiseternal
Feb 24, 2015
This comment has been removed by a moderator.
bluehigh
2.5 / 5 (2) Feb 24, 2015
Space is not constrained to the speed of light, but only energy is so constrained.


Space is a measure, a cognitive evaluation, of distance between objects. If Space expands then, by definition, the distance between objects must also expand.

So, the velocity of Space is constrained by the velocity of the defining objects.

Doug_Huffman
not rated yet Feb 24, 2015
LOL But not the ruler? Pffwwt! It must be really good for you, man!
Gigel
5 / 5 (2) Feb 24, 2015
Movement, just run your experiment and make sure it can make a difference between current theories and your theory. After that tell us how it went. Until then please restrict to one short post at a time. We got the idea.
movementiseternal
Feb 24, 2015
This comment has been removed by a moderator.
PhotonX
4.8 / 5 (8) Feb 24, 2015
Movementiseternal, did you really need 6 posts essentially repeating the same
Thing over and over?do you think if you repeat yourself enough people will believe you or
Take you seriously? ... I save this next part respectfully and mean no offense but instead of
Spamming the same thing over and over again here you should work on your english skills.
This repetitive spamming nonsense has been going on for a while here, in any article mentioning astronomy and cosmology. But Oh glorious day! Now we see: "movementiseternal: This comment has been removed by a moderator." Thank God! Or rather, thank moderator! It's about time this clown had his plug pulled. Hopefully it's site wide and if it is maybe, just maybe, he'll get a clue before opening his next account. I doubt it, but hope springs eternal. Just not movementiseternal.
.
.
swordsman
not rated yet Mar 02, 2015
If space is expanding continually, does that affect the speed of light? If so, then the speed of light must have been much faster at an earlier age, which means that the universe is not as old as has been predicted. But does an expanding universe really affect the speed of light? Only if there is dark matter.

There we go again.
viko_mx
1 / 5 (1) Mar 02, 2015
Space can not travel at all because It is not physical object but geometric consept. Vacuum of cosmic space is a physical object which is transmission medium that defines and control localy or globaly the physical interactions between elementary particles and electromagnetic energy. Therefore it must maintain these properties constant. I do not see how this will happen if can be stretched as turkish delight. But visionaries may have an ingenious solution. I'm looking forward to hear it. They generally do not like reality and need to invent its own reality which fit their mental attitudes. Mathematics and computer simulations can do miracles.
antialias_physorg
not rated yet Mar 02, 2015
If space is expanding continually, does that affect the speed of light?

Unlikely. Light is the yardstick. Also if light had had a different speed then that would have all manner of reprecussions for anything bound by electromagnetic forces (e.g. electron orbitals), which in turn would mean that atoms looked different in the past - and by extension that their spectra would look different. This is not observed.

viko_mx
1 / 5 (1) Mar 02, 2015
The Doppler effect is observed for the propagation of waves in static space. No such effect was observed for expanding vacuum of space because there is no such.
TimLong2001
not rated yet Mar 02, 2015
After we realize that the background red shift is due to photon energy loss from various mechanisms, we can take a closer look at the idea of a universal "speed limit" and see that c, the velocity of light, is a characteristic velocity of E-M radiation and not a limit to possible velocities.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.