The Cosmos


Gravitational Waves From Bubble Universe Collisions?

In the beginning was the bubble: Inflating, in a fraction of a second, from a grain smaller than an atom to a mini-verse the size of a softball. The seeds of all the elaborate particulars of today’s universe, from the vast cosmic web that links galaxy clusters all the way down to the motes of cosmic dust drifting past Earth, were wound up in tiny quantum fluctuations in that original bubble, just waiting for time, and gravity, to uncoil them.

That’s the story of inflation, and it’s the prevailing narrative for how our universe came to be. It has a lot going for it: It explains why the universe has such a uniform temperature, why spacetime appears to be flat, and why physicists can’t find any magnetic monopoles. In the last decade, inflation’s predictions have lined up neatly with observations from telescopes like Planck and WMAP, which have mapped miniscule deviations in cosmic microwave background radiation, an electromagnetic “echo” from near the time of the Big Bang. Though inflation has its critics, it remains the leading theory of how our universe came to be.

Follow inflation to what many theorists think is its logical conclusion, though, and things get very strange. That’s because many versions of inflation lead straight to a multiverse: that is, a cosmos in which our universe is just one of many universes, each with different laws and fundamental constants of physics. The idea is controversial, not least because there is no guarantee that we would ever be able to prove or disprove the existence of these other universes. Now, a team of theorists has shown that a collision between universes would create gravitational waves that could imprint a unique polarization signal on the sky, potentially providing observational evidence for the existence of other universes.

Searching for universes beyond our own. A wood engraving from Camille Flammarion's L'atmosphère: météorologie populaire (1888). Artist unknown. Public domain, via Wikimedia.

Jonathan Braden, who worked on the paper while he was a graduate student at the University of Toronto, compares the multiverse to a pot of simmering water. As the water boils, air bubbles big and small spontaneously pop into existence and jiggle about. Now imagine that our entire known universe is one of those air bubbles, swimming through the “water” of the universe’s native vacuum energy, as other bubbles emerge around it. The analogy isn’t perfect: For one thing, the energy that drives the creation of new bubble universes isn’t thermal energy, like the heat of a stove, but the inevitable fluctuations that are built in to the principles of quantum mechanics. Even stranger, the “pot”—the space in which the bubble universes are emerging—is constantly getting bigger, and the water supply always being replenished.

In this ever-simmering universe, bubbles may occasionally bump in to each other. If our universe was part of such a collision some time in the distant past, it could leave a telltale circular “bruise” on the cosmic microwave background (CMB) radiation. Astronomers first scanned the CMB’s tiny temperature variation for this telltale mark back in 2011, using measurements from NASA’s Wilkinson Microwave Anisotropy Probe, but found nothing. A second analysis also came up empty-handed.

Now, Braden and his collaborators Dick Bond (University of Toronto) and Laura Mersini-Houghton (University of North Carolina-Chapel Hill) argue that, when it comes to finding evidence for bubble universe collisions, those temperature variations may be only half the picture. In a new paper, they show that the collision should also imprint a distinctive polarization pattern in the CMB. Like the signal famously found and lost by the BICEP2 experiment last year, which astronomers initially thought came from the inflation process itself, the polarization would take a form called B-modes, and would be generated by gravitational waves. But, unlike the primordial B-modes from inflation, which should be coming from everywhere at once, the collision signal would be localized to a single disk of sky.

The new prediction comes from considering nuances in the shape of colliding bubbles. Traditionally, researchers have approximated the bubbles as perfect spheres. In reality, though, those spheres would have little bumps and ridges. Braden compares each bubble to a raised relief globe: from far away, it looks perfectly smooth, but up close mountain ranges and valleys become visible.

“Those mountains and valleys begin to grow very quickly once the bubbles collide,” says Braden. In the disk where the two bubbles intersect, “It looks like someone just splatter-painted it–it’s like a Jackson Pollock painting.” Most exciting for scientists, the collision should also produce gravitational radiation. That radiation could be observed today as localized B-mode polarization that matches up with the temperature “bruise” in the CMB.

The odds of picking up such a signal from a telescope like BICEP2, which only observes a small piece of the sky, are low, points out Braden. That’s because the collision, if it happened, would have to be coincident with the telescope’s field of view. “Ideally an all-sky satellite experiment designed to very precisely measure polarization is the best hope,” says Braden. Today, Braden is working at University College London in the laboratory of Hiranya Peiris, who has participated in previous searches for evidence of bubble collisions. He anticipates that she and her colleagues will be eager to take up the search for the new polarization signal.

If the signal is a no-show, that doesn’t rule out the existence of other universes. But if observers do detect and confirm it, it would be revolutionary. “You might not see anything,” says Braden. “But if you do, it’s giving you an observational handle on physics you probably can’t get in any other way.”

Go Deeper
Editor’s picks for further reading

arXiv: Eternal Inflation and Its Implications
Alan Guth, who pioneered the theory of cosmic inflation, provides an in-depth look at its implications for the creation of a multiverse of “bubble” or “pocket” universes.

Early Universe @UCL: Eternal Inflation and Colliding Universes
An introduction to inflation, eternal inflation, and what goes in to the modeling of bubble collisions.

Quanta: Multiverse Collisions May Dot the Sky
Science writer Jennifer Ouellette goes inside the search for evidence of bubble collisions.

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Kate Becker

    Kate Becker is the editor of The Nature of Reality, where it is her mission to blow your mind with physics. Kate studied physics at Oberlin College and astronomy at Cornell University, and spent seven years as senior researcher for NOVA and NOVA scienceNOW. Follow her on Twitter and Facebook.

    • Dervis Mahmutcehajic

      Back again to Higgs field that probably did not exists for fraction of second allowing for inflation. Nothing had a mass . After it was established, the speed of light become the limit.

    • Stephen L

      My confusion is the analogy of a single universe as a soap bubble. This would imply that our universe as “edges.” The agreed on fact though is that wherever you are in the universe, from your perspective you are in the center. So space sort of wraps around itself, which is mind boggling. In a “multi-verse” this would just repeat itself multiple or even an infinite amount of times.

      • MNix

        Think about it like this: the surface of a sphere is a finite unbounded 2-D area. That is, it’s finite in size with points that definitely lie on the surface and points that definitely are outside the surface. However, it does not have a boundary; you can follow the surface of a sphere in any direction forever and never find an edge. The universe is thought to be like a 3-D version of that; finite in scope but without a boundary, embedded in a higher-dimensional “bulk”.

        Of course, that doesn’t even touch on holographic theories of cosmology.

      • Nedward Marbletoe

        the bubble isn’t a separate universe. there’s one universe that has varying inflation, making different bubbles, and the bubbles collide. over all spacetime, with zillions of bubbles, the space wraps around so it’s finite but has no edges.

    • “It explains why the universe has such a uniform temperature” — the universe has such a uniform temperature because it has been here forever (infinity) and that is a long time.

      • Septimius


    • Septimius

      We haven’t even been able to detect gravitational waves in THIS universe. Until that happens talk of a multiverse will continue to be mathematical and philosophical masturbation, just like String Theory.

      • Nedward Marbletoe

        so it’s useless… until it’s tested. that makes no sense. then we should give up making hypotheses. just wait for the proof to fall out of the sky.

      • Robert S. Orr

        You’re right, of course. I think cosmologists are indulging in too much hype. Remember the BICEP-2.? “echoes of Inflation” turned out to be dust.

        • Ronny

          It not too much hype to them, it’s their passion, they see more in it than we do, it’s like a builder finally working out a construction problem in his head, eureka, but he can’t really know till its sorted out.

          • Robert S. Orr

            I mean hyping things that are far from established to the public media.

            • Ronny

              I’d rather listen to scientific hype than political hype, it’s ok to talk about hypothesis, I like them telling us their thoughts.
              Sure some get a little too excited, but that normal, it’s their passion.
              How else can laymen get an ear in on the conversation? I wanna hear their ideas, it gives us an idea of whats going on, what could be.
              If we wait for established theory we may wait a long time to find out stuff we don’t like.

      • Nedward Marbletoe

        btw they aren’t talking about detecting gravity waves, but the results of them on the CMB. in other words, microwave patterns.

      • Erik150x

        Well, I for one am sure glad Einstein and many others got their science rocks off postulating many things which were unproven and even unprovable at the time. Masturbation isn’t so bad. You should give it try.

        • Ronny

          I’ll wait for my other universe to get home, she’s much better.

    • Robert S. Orr

      “If the signal is a no-show, that doesn’t rule out the existence of other universes.”

      I nominate this for the prize of Most Ludicrous, Content Free Statement of the Week.

      • Scruffy J Nerfherder

        It is simply logic. The absence of something doesn’t not preclude the existence of it. Like a road with no tire marks on it does not mean that cars do not exist. Proving the existence of parallel universes with depend on many sources of corroborating evidence. This is only the beginning.

        • Robert S. Orr

          In fact, it is quite the opposite of logic. Their lack of any observation makes no statement whatsoever about parallel universes.

          • Scruffy J Nerfherder

            Well, yes that’s true. Conventional thought holds that while the mathematics indicate that they are a possibility, it is unlikely that direct observations can be made. So we are left with indirect evidence.

      • visibleunderwater

        welcome to the world of string theory! lol

    • “It explains why the universe has such a uniform temperature” — the universe has such a uniform temperature because it has been here forever (infinity) and that is a long time.

      • Erik150x

        If its been there forever where do all the new stars come from? Why haven’t they all burnt out by now? What do you make of the accelerating expansion of the universe? I mean was it just sitting there all nice an static and then suddenly starting expanding? Your view of a universe which has been around forever has a lot of contradictions to account for.

        • The universe being here forever has no effect on star formation. Stuff explodes here, coagulates there.

          There is no expansion. They think that whole Big Bang thing because of the redshift of light. There are plenty of other reasons for red shift. Big Bang is the most far-fetched, BB is the stupid reason.

          NOTE: If I said the Universe has been here forever that means NO BIG BANG. Why would you even ask about expansion? You can’t add things up correctly?.

          If the Universe has been here forever it DOES NOT mean everything is static… that’s more stupid thinking.

          You have the same problem as most others, you can only think one level deep.
          i.e. if one thing changes it effects other things. You are oblivious to stuff like that.

          • Isak

            If the Universe has been here forever then our night sky wouldn’t be dark. As a matter of fact, day and night would appear to glow with bright white light. We wouldn’t be able to make any distinction between the two since all photons traversing the Universe would have had infinite time to reach us. The Universe, although expanding to what seems infinity, has not existed forever. It had a beginning.

            • You have the same problem as most others, you can only think one level deep.

            • Chris

              That is not how light works. If I put myself in a perfectly dark, sealed room, and turn on a flashlight, the room does not get brighter with time. Even if that flashlight could continue shining forever, the total amount of light would not compound on itself. Even given an infinite amount of time the room would not become bright white, or any brighter, in fact than when the light is initially switched on. You are mistaking the physics of light and photons with elements of mass like water. A water hose would eventually fill the room. Light does not. Far from proving your point, you have only illustrated your lack of understanding with regard to the concepts you are trying to use to make a point. Your argument is fallacious.

              The light we do see in the sky is a result of the position of the source of light, and time. Even if it could be said (which is a pointlessly unprovable hypothetical) that over an infinite amount of time, at some point, there would be light shining on the earth from every conceivable angle and direction, that would not make the earth consumed with bright light even in an infinite time and infinite universe model, as the function of time, though infinite, does not cease to exist. Also, light does dilute and get dimmer with distance. You can try this out for yourself. Turn on a light outside. Walk farther and farther away. Observe.

              I find it shocking that you do not have enough curiosity within you to discredit the reasoning you are parroting without having to have it explained to you.

          • disqus_82xRSNWxa4

            I think he’s leaning towards gawd. Or just trolling.

          • Erik150x

            Try a reading a book or two. Learn some science. Your arguments are so ridiculous its pointless to even try to discuss it with you.

            • DUDE! You are an imbecile.

              Here is an easy example of your stupidity…

              “If its been there forever where do all the new stars come from?”

              I even gave you a link…

              NOTE: You are not alone. There are plenty of imbeciles like yourself that think they are “smart” but can only think one level deep.

            • Erik150x

              I understand the theory of star formation perfectly well. But if the universe was around forever, then all the stars would have converted their fuel of lighter elements to iron and there would be nothing left to create stars from. Also gravity would have pulled everything together into one big black hole, even that would have happened by now if not for the expansion of the universe. LOL, but astrophysicists and my self can’t think more than one level deep? You are under a massive delusion about your own intellect. Your ides is that stars just keep forming and exploding is far too simplistic of a model. It doesn’t work like that.

            • You believe everything you were told.
              You are an imbecile / simpleton

            • If we were playing chess you would be able to move one pawn and then you would have to stop. Cannot think any further.

            • According to you all stars got their start at the Big Bang 14 billion years ago?

              How do you explain the Sun?
              It’s only 5 billion years old.

              You are an actual imbecile.

            • Erik150x

              “According to you all stars got their start at the Big Bang 14 billion years ago” – uh, no dumb-ass. Stars are forming all the time. You have no clue what your talking about. Read a book, get a clue.

            • “Stars are forming all the time.” — Yes, so the age of the Universe is irrelevant — it can even be infinity.

              That means your comments are now contradictory.

              You are an imbecile!

            • Erik150x

              They are forming all the time, “right now”. Sorry I can;t fill in all the blanks in your intelligence. Do you understand that stars burn fuel. Fusion reaction changes the elements into heavier elements. The fuel for stars is not endless. Are you really this dumb or just pretending?

            • Imbecile / simpleton

        • Here…

          read up about star formation mr. looney tunes.

    • Look787

      If our universe is a bubble, which I believe it is, that means the universe is not flat. What light we are detecting is such a small area of said bubble. Space is much larger than the human can imagine. And I believe the sphere is the perfect shape.

      • Erik150x

        It’s flat in 4 dimensions (space-time), not in 3.


          off course I agree with you as BRANE MULTIVERSE has many dimensions

      • Isak

        So you think it’s the inflationary multiverse model? Did you know there were three other major types of models? Quilted, quantum, and membrane? I personally like the quantum multiverse hypothesis. It helps to explain particle-wave duality.


          i also like QUANTUM MULTIVERSE as it proves-
          “possibilities are endless” and any of the endless possiblity may be right

          • NILESH SHINDE

            isak do u know about tanki online


          in quantum mechanics there are many possibilities but the outcome is only one. for example-
          suppose you are in a restaurant and ordered something and when a covered dish comes before you, it may have any of the dish from the menu but when youopen it all the possibilities collapse and only one dish that you selected come in front as the outcome


      i suggest you TANKI ONLINE


      i believe in parallel universe-

      • Robert S. Orr


        • You probably believe Big Bang, correct?
          Whatever is in your brain that allows you to believe crazy ideas like BB is also giving him the ability to believe stuff like parallel universe.

          NOTE: sorry if I assumed incorrectly about you believing BB.

          • NILESH SHINDE



            NOTE: i don’t want to oppose, quarrel, mess or disagree with you

            I AM A FRIEND OF ALL


          the answer is so simple as the parallel universe or multiverse theory give the appropriate meaning to understand this universe endless and also limited at the same time
          this quantum multiverse seems to be very real


      according to parallel universe our duplicates are present but in different universe and we cannot reach them as our universe is continuously expanding thus or duplicates are going far from us with great speed. however we and our all duplicates experience different things and not the same

      • Isak

        this would be the quilted multiverse model which is sometimes combined with the inflationary multiverse model


          in quilted multiverse our duplicates are present but too far from our patch
          While inflationary multiverse there are many bubble like universe and laws of physics may be different everywhere but i think mathematics is same everywhere
          off course your statement may be right as there may be any of the billions or endless possibilities

          • NILESH SHINDE

            Another universe might be essentially right next to ours.

    • Patrick Intiso

      Trying to prove an event that cannot be proven with current knowledge and technology. Might as well give credit and cause to the Loch Ness Monster.

      • Erik150x

        But that is exactly the point of this article, is that it may be provable.

      • disqus_82xRSNWxa4

        Many ideas from early scientists took hundreds of years to prove. No reason to give up.

    • The Universe has been here forever, that’s infinity.
      And the size of the Universe is also infinity.

      If you believe anything else like bubbles or higher dimensions you are unbelievably gullible.
      Do you think there was a Big Bang? That means everything was in one spot. Go out and take a look through a telescope and think about that.

      NOTE: I know they say “everything was in one spot but that spot was everywhere” — you probably believe that also?

      • Isak

        If the Universe has been here forever then our night sky wouldn’t be
        dark. As a matter of fact, day and night would appear to glow with
        bright white light. We wouldn’t be able to make any distinction between
        the two since all photons traversing the Universe would have had
        infinite time to reach us. The Universe, although expanding to what
        seems infinity, has not existed forever. It had a beginning…at least in the sense that our current cycle is separate from previous and future reincarnations (if such is the case). However let’s take our telescopes out like you say. Edwin Hubble made measurements and calculations on the distances of galaxies through his telescope. He found that Galaxies were moving away from one another. And further galaxies moved away even faster. The Universe was expanding. Run the clock backward and spacetime contracts. All the energy within its bounds moves closer and closer together. So yes, “everything was in one spot but that spot was everywhere.” It was the birth of the Universe as we know it. Historically speaking, the Multiverse Hypothesis is simply our way of ensuring that our ignorance and ego doesn’t get in the way again. At first it was the Earth who was special and at the center of the Universe. We looked up into the sky and saw other planets. Next it was the Sun who was special. Nope, just one of billions in our galaxy. But surely our galaxy must be the only one? Wrong again. There are billions in the observable Universe. Why should the Universe be alone if it doesn’t seem to make anything only once? The simplest way for you to prove your “infinite Universe” is to show me a sky aglow with white light. That’s all I ask.

        • You have the same problem as most others, you can only think one level deep.

          This is only a guess…
          “As a matter of fact, day and night would appear to glow with
          bright white light.”

          Do you understand? That is a guess.

          Read this carefully: And my reasons are different than that anyway.

        • “everything was in one spot but that spot was everywhere.” — that is ridiculous

    • paperpushermj

      Still doesn’t answer the question of ” What Banged and Why it Banged ?” Then We can move on, to did it only happen once or multiple times.

      • Isak

        Well that’s the trillion dollar question, isn’t it?

    • disqus_82xRSNWxa4

      What if we’re all just in the Matrix? Mind Blown!!!


        May be.
        As there are so many theories about the universe and it is not important that only one of them may be correct
        By the way i believe in the parallel universe theory

      • Dennis Ludwig

        Wow… no originality whatsoever.

        • disqus_82xRSNWxa4

          Look who’s talking.

          • Dennis Ludwig

            STOP IT! You are blowing everyone’s mind.

    • Helmut Schimpfke

      How about a “Big Flush” from-to another universe via black holes.

    • Gregory Trane

      I accept the theory of multiple universes and parallel universes within one universe. Could ruptures of branes that seperate universes cause infation or the “big bang”?

    • visibleunderwater

      No evidence? What about this:

      And this was back in 2010…