Cosmology in the Year 1 Trillion

Article written: 23 Jun , 2011
Updated: 24 Dec , 2015
by

[/caption]Much of what is known today about the birth of the cosmos comes from astronomical observations at high redshifts. Due to the accelerated expansion of the Universe, however, astronomers of the future will be unable to use the same methods. In a trillion years or so, our own Milky Way galaxy will have merged with the Andromeda galaxy, creating a new galaxy that has been quaintly termed “Milkomeda.” All of our other galactic neighbors will have long disappeared beyond our cosmological horizon. Even the CMB will have been stretched into invisibility. So how will future Milkomedans study cosmology? How will they figure out where the Universe came from?

According to a paper published by the Harvard-Smithsonan Center for Astrophysics, these astronomers will be able to decode the secrets of the cosmos by studying stellar runaways from their own galaxy: so-called hypervelocity stars (HVSs). HVSs originate in binary or triple-star systems that wander just a hair too close to their galaxy’s central supermassive black hole. Astronomers believe that one star from the system is captured by the black hole, while the others are sent careening out of the galaxy at colossally high speeds. HVS ejections occur relatively rarely (approximately once every 10,000-100,000 years) and should continue to occur for trillions of years, given the large density of stars in the galactic center.

So how would HVSs help future astronomers study the origins of the Universe? First, these scientists would have to locate an ejected star beyond the gravitational boundary of Milkomeda. Once beyond this boundary (after about 2 billion years of travel), the acceleration of a HVS could be attributed entirely to the Hubble flow. With advanced technology, future astronomers could use the Doppler shift of its spectral lines and thus deduce Einstein’s cosmological constant and the acceleration of the Universe at large. Next, scientists could use mathematical models of galaxy formation and collapse to determine the Universe’s mass density and age at the time that Milkomeda formed. From their knowledge of the galaxy’s age, they would be able to tell when the Big Bang occurred.

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20 Responses

  1. Well, this scenario is just a possibility, among others. Although it might currently seem the most plausible one, you have to assume the Universe will keep expanding at an accelerated rate, driven by dark energy.
    As there are still many questions about dark energy’s equation of state, whether it will keep behaving the way it does now or not, or even about its existence, all this relies on quite a big assumption.
    But it is quite fascinating, anyway!

    • Torbjörn Larsson says

      Now you are triggering one of my pet peeves. Theories like standard cosmology doesn’t work that way at all.

      An assumption is an ad hoc attempt at explanation. Formally it is an example of abductive reasoning.

      If you think about it carefully you will find that scientists don’t do much of that, I believe; though colloquially the term is still passed around.* From a formal view scientists can use inductive reasoning to great advantage to propose putative hypotheses. But more importantly deductive reasoning is used when testing hypotheses – does it work or not?

      Any among explanations are valid simultaneously so amounts to story telling. Hypotheses narrows the field considerable because they are asked to be testable. And theories go whole hog by many tests combined with competition; there can only be one winner, and it has passed a needle’s eye as bottleneck for the testing.

      Speaking of which, late-time integrated Sachs-Wolfe effect has ruled out no dark energy theories. In the same way that we have ruled out “no Mercury” as behind the observation of light that curiously behaves like a planet called Mercury reflects it, we have ruled out “no dark energy” as behind the observation of light that curiously behaves like dark energy sets an imprint on it.

      ———————-
      * If you wish you can trace it formally as well. It can very well start out as an assumption, instead of seeing a pattern you have a hunch – physics sense top down instead of data driven bottom up.

      But as soon as you ask how to verify it, it will be subsumed into a hypothesis.

      • An assumption is a hypothesis that is taken for granted, and that is exactly what we have here. Dark energy is taken for granted, so the Universe will look like that (the description given above) in a trillion years.
        And, even though they might not be very attractive, “no dark energy” theories have not been ruled out at all yet. The authors of the paper you linked to mention inconsistencies between BAO and their results. They also mention that considerable uncertainties remain (and yet they claim this is important evidence for the existence and nature of dark energy…).
        Moreover, some observations seem to point that there might be something wrong with the standard model.
        We are still pretty far from the certainty that dark energy exists, and all the alternatives haven’t been ruled out yet.

      • Anonymous says

        The observations of an accelerated expansion are in line with the de Sitter configuration of spacetime for the universe. The presence of matter perturbs this solution some, but for brevity I will not delve into that. The scale factor which gives the sliding away of points of space is exponential in its growth

        a(t) ~= C exp(sqrt{?/3} t),

        which corresponds to an Einstein field equation

        R_{ab_ – (1/2)Rg_{ab} – ?g_{ab} = 0

        The cosmological constant term ?g_{ab} which drives this accelerated expansion is then equated to a stress-energy tensor ?g_{ab} = (8?G/c^2)T_{ab} so

        ? = (8?G/c^2)?.

        This density ? is determined by the quantum vacuum or ? ~ for ? a quantum field and H the Hamiltonian or “quantum energy operator,” and where this is evaluated on the vacuum state |0>. .

        What is known is that the universe accelerates. Observations of this acceleration by looking at the luminosity of supernova IIa gives decent evidence for a de Sitter spacetime structure. This part is pretty well in agreement with observations. The “dark energy” comes in with the equating of this term to a vacuum energy density ? ~ . The big question is then what is the quantum vacuum which determines this. This is where the “dark” comes in. In effect to understand this dark energy it requires that we know what the quantum eigenstates of the universe are. To work this out requires an understanding of quantum gravity or quantum cosmology.

        Barring some unknown thing lurking under the waterline, the universe will exponentially expand indefinitely. In 10 billion years most other galaxies will no longer be visible in the optical band, but will require infrared detection. The CMB will also be detectable, but at a temperature of 1.5K. So in just double the current age of the universe the appearance of things will be quite different. Any ETI which exists in this future range may be able to conduct astronomy and probably infer much of what we have about the observable cosmos. There will doubtless be G and K class stars around which may support planet comparable to Earth. There will also be red dwarf stars, and maybe ETI can evolve on a planet similar to Gliese g (g being the designation as I recall). In 100 billion years things will be decidedly darker and colder, with stars that are almost entirely red dwarfs and the universe will have expanded to much larger redshift values, and around 1 trillion years galaxies may be facing the end of their star formation period.

        We exist in a sort of sweet spot, where if we came about 5 billion years ago it is unlikely we would detect the accelerated expansion, for matter would still have an appreciable gravitational hold. After about 10 billion years I think it will become tough for any EIT to put together a reasonable model of the universe. This time frame is also coincident with the occurrence of higher metallicity in the universe after about 5 billion years ago, and what might be the end of abundant star formation in 10 billion years. There will after this time be few G-class stars such as the sun. I doubt there will be ETI anywhere in 1 trillion years.

        LC

  2. Danny Nye says

    I thought the Milkomedians would browse through the U T archives like I do. 😉

  3. Anonymous says

    The universe outside of Milkomeda will start to redshift from the optical band in 10 billion years. At that time other galaxies will be visible in the IR and the CMB could be detected in the radio frequency domain with a temperature of .1C. In ten trillion years other galaxies might be picked up by long wavelength radio-scopes. The CMB would require a ¼ wave stack larger than a planet, though in principle it could be detected.

    We might want to ponder what the Milkomeda galaxy will look like and I suspect it will be red and dim. Most stars will be red dwarf stars, with very few bright stars. So beings on some planet might find the sky to be utterly black, with maybe a few reddish nearby stars visible. Things will be winding down.

    LC

    • Anonymous says

      .1C or .1K ?

      • Anonymous says

        It is Kelvin of course. I was also off by an order of magnitude. Using Wein’s law the wavelength of radiation will be about ten million times what it is now. So estimating the CMB radiation ~ 1cm on average this is 10^8 cm or about a thousand kilometers. It would require one huge antenna to pick that up.

        LC

  4. WaxyMary says

    Welcome Vanessa d’Amico, interesting formulation you have interpreted.

    Mary

  5. Anonymous says

    “First, these scientists would have to locate an ejected star ..”

    Surely they would have been tracking them. We already watch the core carefully so should be able to predict the close approach of binaries and monitor that process. Tracking ejectees through the galaxy would give valuable information about the dark matter distribution and their subsequent paths would be very well characterised.

  6. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

  7. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

  8. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

    • Torbjörn Larsson says

      Galaxies are gravitationally bound, and therefore not expected to expand with the universe.

      Where the boundary goes is debated. It seems some observations may indicate parts of the Local Group is expanding. Some as here claims that only isolated galaxies will remain as bound non-expanding systems. And I have seen some claim that clusters are bound in this sense.

      If the universe expands but not a part of it, that part will also be cut of from indefinite expansion by aggregating galaxies.

      So instead of the article saying that galaxies are expanding indefinitely it says galaxies are not expanding. That is the setting for studying expansion by other means as described in the article (using ejected stars).

      • Anonymous says

        The scale factor for the universe grows as a(t) = C cosh(sqrt{?/3} t) or a(t) ~= C exp(sqrt{?/3} t) for t > sqrt{3/?} . That function is exponential and applies to all time derivatives of the scale factor. So at some time in the future galaxies will indeed be ripped apart. However, this is not for a long time in the future. For the time t near sqrt{3/?}, we can say there is on “efold,” of the acceleration. The acceleration is then

        a(t) = (C?/3) exp(sqrt{?/3} t)

        ? =~ 10^{-36}sec^{-2}

        So that over the dimension of a galaxy C =~ 10^{23}cm the acceleration is 10^{-13}cm/s^2. The angular velocity of a galaxy is ? ~ 2×10^{-13}sec^{-1} and the acceleration a = ?^2r of a star such as the sun is about 4×10^{-3}cm/sec^2 in it orbit around the galaxy. So it will require about 23 efolds for the acceleration of the universe to begin to tug stars such as the sun away from its galaxy. This will happen in about 10^{20} years.

        This is not the big rip scenario. That has acceleration increasing to “infinity” at some finite time, as a curve asymptotes to a vertical line.

        LC

  9. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

  10. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

  11. Kenneth Roldan Tello says

    this is one of the universe’s greatest mysteries and a clear evidence that galaxies are expanding indefinitely. Milkomedia galaxy could be humongous in terms of size, but this phenomenon could pave a way for the scientist to fully unveil the origin of the universe.

    It could take years, or even billion of years to reach the boundaries of this newly formed galaxy, that time we are no longer part of that discovery.

  12. John Acton says

    google

  13. astrobites says

    Universe Today readers may also be interested in the following related links:

    An Astrobites summary of this paper from February: http://astrobites.com/2011/02/02/astronomical-methods-for-the-year-1-trillion/

    A follow-up conversation between the paper’s author (Avi Loeb) and Freeman Dyson: http://astrobites.com/2011/02/03/avi-loeb-and-freeman-dyson-on-the-future-of-the-universe/

    The official press release from the CfA in April: http://www.cfa.harvard.edu/news/2011/pr201111.html

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