More Cosmic Tension?

Quite a lot of fuss was being made in cosmological circles while I was away last week concerning a paper that had just been published in Nature Astronomy by Eleonora Di Valentino, Alessandro Melchiorri and Joe Silk that claims evidence from the Planck Cosmic Microwave background and other data that the Universe might be closed (or at least have positive spatial curvature) in contrast to the standard cosmological model in which the spatial geometry is Euclidean. Nature Astronomy is behind a paywall but the paper is available for free on the arXiv here. The abstract reads:

The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard ΛCDM model. A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than 99% C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed universe, discordances higher than generally estimated arise for most of the local cosmological observables, including BAO. The assumption of a flat universe could, therefore, mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent. Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics, or simply are a statistical fluctuation.

I think the important point to take from this study is that estimates of cosmological parameters obtained from Planck are relatively indirect, in that they involve the simultaneous determination of several parameters some of which are almost degenerate. For example, the `anomalous’ lensing amplitude discussed in this paper is degenerate with the curvature so that changing one could mimic the effect on observables of changing the other; see Figure 2 in the paper.

It’s worth mentioning another (and, in my opinion, better argued) paper on a similar topic by Will Handley of Cambridge which is on the arXiv here. The abstract of this one reads:

The curvature parameter tension between Planck 2018, cosmic microwave background lensing, and baryon acoustic oscillation data is measured using the suspiciousness statistic to be 2.5 to 3σ. Conclusions regarding the spatial curvature of the universe which stem from the combination of these data should therefore be viewed with suspicion. Without CMB lensing or BAO, Planck 2018 has a moderate preference for closed universes, with Bayesian betting odds of over 50:1 against a flat universe, and over 2000:1 against an open universe.

Figure 1 makes a rather neat point that the combination of Planck and Baryon Acoustic Oscillations does not separately give consistent values for the Hubble constant and the curvature and neither does the combination of Planck and direct Hubble constant estimates:

I don’t know what the resolution of these tensions is, but I think it is a bit dangerous to dismiss them simply as statistical flukes. They might be that, of course, but they also might not be. By shrugging one’s shoulders and ignoring such indications one might miss something very fundamental. On the other hand, in my opinion, there is nothing here that definitely points the finger at spatial curvature either: it is possible that there is something else missing from the standard model that, if included, would resolve these tensions. But what is the missing link?

Answers on a postcard, or through the comments box.

15 Responses to “More Cosmic Tension?”

  1. I realize the suggestion is about as popular as a skunk at a garden party, but when will it be possible to look beyond the idea of an expanding universe?
    For one thing, if the overall curvature is flat, what is left to expand? Yes, the space between galaxies expands, but at a rate equal to which it collapses into them. Wouldn’t some sort of cosmic convection cycle be worth considering? Then Hubble actually discovered evidence of a Cosmological Constant. The balance to gravity.
    It’s also pretty much impossible to falsify the Big Bang Theory, as any time there is a gap between prediction and observation, some enormous, otherwise invisible force of nature is plugged in and all is well. What if accountants could just write in a figure and call it dark money, whenever they found gap in the books?
    The first patch was when they realized the redshift increases proportionally in all directions, creating the effect that we appear to be at the center of this expansion, so it was changed from an expansion in space, to an expansion of space, based on spacetime.
    Which would seem to overlook the essential premise of GR, that the speed of light is always measured as a Constant. If the light is being redshifted, because it is taking longer to cross an expanding universe, than the speed of intergalactic light is not Constant to intergalactic space!
    Two metrics are being derived from the same light. One based on the spectrum and one based on the speed. Since the speed is still assumed to be the denominator, given this expansion is relative to it, it would make the expansion the numerator and so it would still be increasing distance, measured in a stable, light speed based metric, not an expansion of the space based on that metric.
    We are at the center of our point of view, so possibly an optical effect might eventually be worth looking into.
    Here is a paper I came across some years ago, pointing out that while single spectrum light only redshifts because of recession, multi-spectrum light “packets” redshift over distance, as the higher spectrums dissipate faster;
    h ttps://fqxi.org/data/forum-attachments/2008CChristov_WaveMotion_45_154_EvolutionWavePackets.pdf
    (I left a gap between h and ttps, so the whole paper doesn’t download.)
    Which goes to the issue of whether photons are ultimately irreducible, or not, and whether we are actually sampling a wave front, not individual photons, traveling billions of lightyears.
    In which case, the cosmic background radiation would be the light of ever more distant sources, shifted off the visible spectrum. The solution to Olber’s paradox.
    Only time and the James Webb will tell.

    • I have rarely seen more misconceptions in so little text. If you are seriously interested in cosmology, check out Harrison’s excellent textbook Cosmology: The Science of the Universe. Although there is no lack of detail, it is accessible without much mathematics, and he spends much time debunking common misconceptions.

      • Thanks for the reply. I seem to be full of misconceptions and not entirely sure why they chose to crowd my head;
        View at Medium.com
        As for this particular issue, I’m going to wait for the JWST to refute it.

      • Please explain, in principle, what a JWST refutation could look like.

      • That the finer detail of the cosmic background would point towards ever spatially further and distributed sources of light and not as some form of singularity.
        Here is a post Zeeya Mirali put up some years ago, where I’ve added links over the years, of examples of cosmic phenomena that are difficult to shoehorn into the time frame and overall premise of the BBT.
        http://fqxi.org/community/forum/topic/1578
        Did you look at the initial link I posted, about light “packets?”
        Off to work at the moment, so I’ll be back later….

      • If you mean the cosmic microwave background, JWST will not measure it.

      • That the finer detail of the cosmic background would point towards ever spatially further and distributed sources of light and not as some form of singularity.”

        Show me your calculation of the expected power spectrum in your theory.

      • Admittedly I’m not much of a mathematician, but as I’m fond of pointing out, epicycles were brilliant math, for their day. In fact, far more accurate than early heliocentric models. Yet they were lousy physics, not because every detail hadn’t been worked out, but because underlaying assumptions were biased.
        Not to go too far off topic, but I think one of the main biases built into current theory is treating time as the narrative dimension, past to future, measured as units of duration, when the reality is that change turns future to past. Tomorrow becomes yesterday, because the earth turns. Duration is this physical state, as events coalesce and dissolve, future to past.
        So there is no physical dimension of time, as the past is consumed by the present, in order to inform it, aka, causality and conservation of energy.
        Time is asymmetric, because what is being measured, a particular action, is inertial. The earth only turns one direction. Entropy is not measured as time and is a second order effect of the flux/complexity of change.
        Consequently time, as effect, is similar to temperature, pressure, color, etc. Think frequencies and amplitudes.
        It is just that as mobile organisms, our sequential process of perception is foundational to our understanding of the larger reality.
        Potential, actual, residual.
        Tying this back into the cosmology, the energy is conserved, so the process goes past to future, while the patterns generated go future to past. Consciousness goes past to future, while thoughts go future to past. So we perceive the patterns, ignoring the process generating them and doing the perception.
        As the primary fact of galaxies is that while energy radiates out, mass coalesces in. Thus the process and the patterns are going opposite directions.
        Since we cannot consciously perceive energy, or it would whiteout, only the patterns generated, we are constantly trying to quantify this dynamic, so yes, it is information all the way down, but it is energy all the way up.
        It’s just our digestive, respiratory and circulatory systems processing our energy, not the nervous system. We are like fish, trying to understand water.

  2. “the Universe might be closed (or at least have positive spatial curvature) in contrast to the standard cosmological model in which the spatial geometry is Euclidean”

    Two points. First, spatial closure and positive curvature imply each other unless there is a non-trivial topology. Second, it depends on what you mean by the standard model. If you mean simply the cosmological parameters, then the uncertainties have always been such that positive spatial curvature is allowed. Second, if you mean some prediction of inflation, then this implies merely that the radius of curvature is very large, so that the universe is very close to spatially flat, but doesn’t say anything about the sign of the curvature.

    Where’s the tension? In other words, one can’t demand a 5-sigma result in order to claim a discovery yet worry about 2.5-sigma “tension”. 😐 Either every 2.5-sigma result should be investigated because it might be interesting, or a higher threshold should be used for all. What does it mean to say that there is tension? Is it a way of trying to claim a discovery without actually saying so, so that one doesn’t lose face whatever the ultimate outcome?

  3. If the radius of curvature is not for all practical purposes infinite, the inflation people should come up with a prediction before it is measured precisely or else risk losing credibility. I remember “open inflation” (back when the low-density universe could no longer be ignored, thanks in part to Coles and Ellis, but most people hadn’t yet accepted the cosmological constant). While contrived, it did create the impression that, whatever the observational data, there is a theory of inflation to match. 😐

  4. “If you mean the cosmic microwave background, JWST will not measure it.”

    From the Webb site;
    “Why is a powerful infrared observatory key to seeing the first stars and galaxies that formed in the universe? Why do we even want to see the first stars and galaxies that formed? One reason is… we haven’t yet! The microwave COBE and WMAP satellites saw the heat signature left by the Big Bang about 380,000 years after it occurred. But at that point there were no stars and galaxies. In fact the universe was a pretty dark place.”

    My prediction then, is pretty clear. That upon further examination, those very faint variations in the cosmic background radiation are not just the fuzzy remnant of the Big Bang, but start to become distinct sources of radiation, potentially with the spectral signatures of heavier elements, just shifted far down the spectrum.

    • Phillip Helbig Says:

      This has already been ruled out decades ago.

      • Yes, but what has been ruled in? Multiverses? Looking at super symmetry and string theory, it’s not impossible for theory to go out on a limb. Trial and error. To say something decided decades ago stands, no matter what is discovered, isn’t science. Presumably we will continue to discover the unexpected.
        To go back to my original point, that two metrics of space are based on the same light, the only rebuttal anyone has given me is that light is measured locally, while space expands universally. To which I’ve responded that it has to expand locally, in order to expand universally.
        To use the inchworm on an expanding balloon analogy, the balloon has to expand under the inchworm as well and that constitutes two metrics being compared to one another. What is the basis of the rate the inchworm/speed of light moves, if not intergalactic space? If it was expanding intergalactic space, wouldn’t the rate of the inchworm/speed of light have to increase, in order to remain Constant?
        Observing the battles over string theory, from Woit’s blog, it is not that I’d expect cosmologists to question ideas they have devoted their lives to, either, but are all future generations of cosmologists required to chase after premises that are beyond testing, such as multiverses?
        Let’s just say, I think the Overton window of cosmological speculation needs to be somewhat more open.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: