Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Is Inflation unlikely or likely?

  1. Oct 16, 2017 #1
    Sabine Hossenfelder and others think it’s unlikely because the latest Planck Satellites didn’t support some observations such as:



    “Notably, if we knew inlation had occurred, there is one feature
    we could be fairly certain of inding in the Planck CMB
    observations because it is common to all the simplest forms of
    inlationary energy, including those presented in standard textbooks.
    At the same time that quantum luctuations produce
    random variations in inlationary energy, they also produce random
    warps in space that propagate as waves of spatial distortion
    across the universe once inlation ends. These disturbances,
    known as gravitational waves, are another source of hot and
    cold spots in the cosmic microwave background radiation, albeit
    ones that have a distinctive polarizing efect (that is, the gravitational
    waves cause light to have a certain preferred orientation
    for its electric ield, depending on whether the light comes
    from a hot or cold spot, or some place in between).

    Unfortunately, the search for inlationary gravitational waves
    has not panned out. Although cosmologists irst observed hot
    and cold spots with the COBE (COsmic Background Ex plorer)
    satellite in 1992 and with many subsequent experiments, in -
    cluding even more recent Planck satellite results from 2015,
    they have not found any signs of the cosmic gravitational waves
    expected from inflation, as of this writing, despite painstaking
    searches for them. (On March 17, 2014, scientists at the BICEP2
    experiment at the South Pole announced the detection of cosmic
    gravitational waves but later retracted their claim when
    they realized they had actually observed a polarization effect
    caused by dust grains within the Milky Way.) Note that these
    expected cosmic gravitational waves have nothing to do with
    the gravitational waves created by merging black holes in the
    modern universe found by the Laser Interferometer Gravitational
    wave Observatory (LIGO) in 2015.”

    May I know your thoughts and arguments about this?
  2. jcsd
  3. Oct 16, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    I don't think this is accurate at all.

    The "smoking gun" for inflation is indeed these primordial gravity waves. The problem with the above is that it was always highly unlikely that Planck would be able to detect them.

    Planck was never designed to measure polarization. It was designed, first and foremost, to measure temperature differences across the sky to very high sensitivity. The satellite took a very long time to get off the ground because it is quite a complicated telescope design. If I recall, it actually started development at around the same time as the WMAP satellite which launched 8 years earlier. Polarization was added to the design later in development, and the specific method they used to add polarization measurements is prone to systematic errors. Modern balloon experiments do a far better job of measuring the polarization of the CMB (but they have other problems due to the fact that they have to look through Earth's atmosphere).

    As a result of this, early simulations for the Planck satellite indicated that while it might measure primordial gravity waves, we'd have to get extremely lucky for it to do so successfully. Even if it were able to measure them, there was a very good chance that the signal would be too faint for a definitive measurement.

    Planck does put some constraints on inflation, however. It now looks like many specific inflation models may not fit the data. But many other inflation models are still in the running.

    It's very technical, but here's one paper that did an overview of the inflationary models that were still in the running after the first Planck data release:

    And here's an update by the primary author of the above that discusses the state after the 2015 release:
  4. Oct 17, 2017 #3
    I spoke to a scientists who worked on Planck and WMAP. She told me before Planck data was in, that they hoped to measure gravitational waves from inflation but that this was highly unlikely even if they are there. So Planck not finding them is not a problem for inflation. Its a bit like saying you hope to win the lottery and then when you find out you haven't you demand some special explanation as to why not. This is just silly.
  5. Oct 18, 2017 #4
    But do you know that one physicist shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of 10 to the googol (10 to the 100) power!


    "In order to work, and as pointed out by Roger Penrose from 1986 on, inflation requires extremely specific initial conditions of its own, so that the problem (or pseudo-problem) of initial conditions is not solved: "There is something fundamentally misconceived about trying to explain the uniformity of the early universe as resulting from a thermalization process. [...] For, if the thermalization is actually doing anything [...] then it represents a definite increasing of the entropy. Thus, the universe
    would have been even more special before the thermalization than after."[140] The problem of specific or "fine-tuned" initial conditions would not have been solved; it would have gotten worse. At a conference in 2015, Penrose said that "inflation isn't falsifiable, it's falsified. […] BICEP did a wonderful service by bringing all the Inflationists out of their shell, and giving them a black eye."[6]

    A recurrent criticism of inflation is that the invoked inflation field does not correspond to any known physical field, and that its potential energy curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. Paul Steinhardt, one of the founding fathers of inflationary cosmology, has recently become one of its sharpest critics. He calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either.... Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation ... Other configurations lead to a uniform, flat universe directly – without inflation. Obtaining a flat universe is unlikely overall. Penrose's shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of 10 to the googol (10 to the 100) power!"[4][113] Together with Anna Ijjas and Abraham Loeb, he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the Planck satellite.[141][142] Counter-arguments were presented by Alan Guth, David Kaiser, and Yasunori Nomura[143] and by Andrei Linde,[144] saying that "cosmic inflation is on a stronger footing than ever before".[143]"
  6. Oct 18, 2017 #5


    User Avatar
    Science Advisor
    Gold Member

    This isn't an argument against inflation. It's an argument that one of the specific features of our universe that inflation claims to explain (spatial flatness) may not be correct. This weakens the justification for proposing inflation as a theory, but doesn't really overturn the current experimental evidence. Additionally, this kind of argument is very weak for the simple reason that many of the assumptions used to generate the argument are not experimentally verifiable.
  7. Oct 18, 2017 #6
    Has any cosmology scientist suggest that not only inflation didn’t occur, the big bang didn’t occur too and we were still inside the singularity where its infinite computation powers can manifest any imagination and simulate any condition and create virtual environment (which for example produced the cosmic microwave background radiation, etc.)? Has Stephen Hawking ever thought of this in his perpetual suspended animation state in his wheelchair?
  8. Oct 18, 2017 #7


    User Avatar
    Gold Member

    This is pure unadulterated nonsense, so no, I doubt anyone has proposed it.
  9. Oct 19, 2017 #8


    User Avatar
    Science Advisor
    Gold Member

    This is basically the concept of "metaphysical solipsism". There are two main lines of argument that I've seen scientists take on this sort of thing:
    1. If metaphysical solipsism were true, then nothing would matter. So might as well assume some form of realism/naturalism and go on with our lives. According to this argument, worrying about solipsism is a waste of time.
    2. If metaphysical solipsism were true, then the natural expectation would be that there would be no reliable patterns. For example, the address to your house would change every time you tried to get home. You can actually use techniques that exploit this to determine whether or not you're dreaming: if you look at some text in a dream, look away, then look back, the text will almost always have changed. In the real world, this is rarely the case, and even if the text does change there's usually a simple explanation (e.g. it's a computer screen or TV subtitles).
  10. Oct 19, 2017 #9


    Staff: Mentor

    The OP question has been addressed, and the thread is degenerating into speculation. Thread closed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook