• Question: The phrases “Big Yawn”, “Big Crunch” and “Big Ride” are used in my Physics textbook when discussing the future of the Universe. Are these commonly used phrases? What do they mean?!

    Asked by Mrs M to Susan, Scott, Savannah, Philippe, Meirin, Jose, Harrison, Edoardo, David, Anne, Adam on 7 Nov 2019.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 7 Nov 2019:


      Big Yawn and Big Crunch are pretty common. I’ve never seen “Big Ride” before: are you sure you don’t mean “Big Rip”?

      The Big Crunch occurs if the expansion of the Universe eventually slows down and reverses, so that the Universe contracts back into an infinitely hot, infinitely dense state (well, probably not either infinitely hot or dense, but to understand what happens at extreme density you need a theory of quantum gravity, which we do not yet have), like that existing before the Big Bang. In older books I’ve seen this called the Gnab Gib, which is perhaps more explanatory! Experimental data strongly disfavour this as an end-point, unless the current value of dark energy (which tends to push the Universe apart) is decreasing with time – we do not have very good observational constraints on this yet.

      The Big Yawn is a straightforward extrapolation of what’s happening at the moment: the Universe continues to expand at an ever-increasing rate, until nothing beyond our own local gravitationally bound bit of the Universe can be seen; the stars gradually run out of fuel and die; eventually there is nothing left but black holes, neutron stars and cold white dwarfs. This is sometimes called the Heat Death.

      The Big Rip occurs if dark energy strengthens with time: it eventually overcomes gravitational attraction altogether, so that first galaxies, then stars and finally even atoms get ripped apart, leaving the Universe as a sea of individual elementary particles.

      To stand any chance of figuring out which of these is most likely, we need to understand whether – and if so, how – dark energy changes with time. There are proposals for future observations that may be able to shed light on this.

    • Photo: Scott Lawrie

      Scott Lawrie answered on 8 Nov 2019:


      I completed my Cosmology degree over a decade ago never once have I heard the terms ‘Big Yawn’ or ‘Big Ride’. I suspect they are meaningless buzz-words thought up recently to appeal to quick media articles. Having a poke around, I’ve learnt that ‘Big Yawn’ is meant to imply that it will be very boring in trillions of years after the heat death of the universe. Not a very scientific term! Similarly ‘Big Ride’ is meant to imply that the universe could be like a rollercoaster, going up and down faster and slower. Again: not very helpful or descriptive to actual cosmologists.

      At least ‘Big Crunch’, ‘Big Rip’, ‘Big Freeze’, ‘Heat Death’ etc describe actual posssible scenarios!

    • Photo: Anne Green

      Anne Green answered on 8 Nov 2019: last edited 8 Nov 2019 5:48 pm


      I work on cosmology. We use Big Crunch and Big Rip (I’m guessing that Big Ride was a typo), albeit not on a day-to-day basis. But I’d never heard of Big Yawn before!
      _

      All of these terms refer to the long term fate of the Universe. Which depends on what it’s made of and its geometry.
      _

      If the universe is only made of matter and radiation then its geometry (which depends on the total energy density) determines its destiny. If the density is bigger than a critical value the universe is closed: the expansion slows down, eventually stops and then it recollapses in a ‘Big Crunch’. If the density is equal to the critical density then the geometry is flat and the Universe expands forever, but the speed decreases and tends to zero. If the density is smaller than the critical density the universe is open and expands forever at a constant speed. In the later two cases the temperature of the universe tends to zero. This is sometimes called a ‘Big Freeze’.
      _

      This all changes if the universe contains dark energy, an exotic form of energy (rather than matter) which causes the expansion of the universe to speed up rather than slow down. The dark energy could be a cosmological constant (the vacuum energy of empty space), or it could be something more exotic. One of the most exotic possibilities is phantom dark energy, which has a density which increases (instead of decreasing or staying constant) as the universe expands. In this case (mathematically at least!) the expansion rate tends to infinity and everything (galaxies, stars, atoms…) in the universe would be destroyed in a ‘Big Rip’.
      _

      Cosmological obervations (of the cosmic microwave background radiation and type 1a supernovae) tell us that we live in a Universe that is very close to flat (it could be exactly flat, or slightly closed or slightly open). And at some-point in the relatively recent past (by cosmological standards: a few billion years ago) the expansion started accelerating. Detailed studies of the expansion rate of the Universe find that the dark energy density is very close to constant, but it could be increasing or decreasing slightly with time. So we don’t know for sure exactly what our Universe will do in the distant future. To work this out we’d have to understand the physics behind dark energy, and that’s a very difficult open problem.

    • Photo: Harrison Prosper

      Harrison Prosper answered on 11 Nov 2019:


      Cosmologists, like particle physicists, are fond of silly names for things. But, at least the cosmologists choose silly names that hint at the underlying process: Bang Bang, Big Crunch, and Big Rip. Big Yawn escaped me!

      Since Big Rip was mentioned, you may be interested in an article that a former student of mine, Rutger Dungan, wrote some year ago,

      Varying-G cosmology with type Ia supernovae
      Rutger Dungan and Harrison B. Prosper
      American Journal of Physics 79, 57 (2011); https://doi.org/10.1119/1.3486585

      which contains an amusing model of the universe that predicts it will suffer a Big Rip in about 6 billion years from now, at about the same time that the Sun becomes a red giant and at about the same time Andromeda and the Miky Way collide. What an exciting future for our very distant descendants!

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