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

Quantum cosmo test prospects (sampling the literature)

  1. Nov 2, 2008 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Over the past several years research in nonsingular cosmology, particularly LQC bounce models, has appeared at an increasing rate. Of particular interest are those papers suggesting ways that quantum cosmology bounce models might be tested. Since June 2008 enough papers have appeared that I thought it might be interesting to list them and try to get an overview. What kinds of ideas are being brought forward? Which, if any, seem applicable near-term? Which ideas suggest more remote, or even very unlikely, prospects?

    Since a review paper appeared in June 2008:
    http://0806.0339 [Broken]
    I will not go back before that, assuming that the review covers earlier research adequately. And I will mention one one other June 2008 paper, the rest having appeared in July or later:

    Anomalous CMB polarization and gravitational chirality
    Carlo R. Contaldi, Joao Magueijo, Lee Smolin
    5 pages (Submitted on 18 Jun 2008)

    "We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation."

    Loop Quantum Cosmology corrections to inflationary models
    Michal Artymowski, Zygmunt Lalak, Lukasz Szulc
    16 pages, 1 figure
    (Submitted on 1 Jul 2008)

    "In the recent years the quantization methods of Loop Quantum Gravity have been successfully applied to the homogeneous and isotropic Friedmann-Robertson-Walker space-times. The resulting theory, called Loop Quantum Cosmology (LQC), resolves the Big Bang singularity by replacing it with the Big Bounce. We argue that LQC generates also certain corrections to field theoretical inflationary scenarios. These corrections imply that in the LQC the effective sonic horizon becomes infinite at some point after the bounce and that the scale of the inflationary potential implied by the COBE normalisation increases. The evolution of scalar fields immediately after the Bounce becomes modified in an interesting way. We point out that one can use COBE normalisation to establish an upper bound on the quantum of length of LQG."

    Gravitational waves from the Big Bounce
    Jakub Mielczarek
    19 pages, 9 figures
    (Submitted on 4 Jul 2008)

    "In this paper we investigate gravitational waves production during the Big Bounce phase inspired by the Loop Quantum Cosmology. We consider the influence of the holonomy corrections to the equation for tensor modes. We show that they act like additional effective graviton mass, suppressing gravitational waves creation. However, this effects can be treated perturbatively. We investigate the simplified model without these corrections and find its exact analytical solution. For this model we calculate a spectrum of the gravitational waves from the Big Bounce phase. The obtained spectrum decreases to zero for the low energy modes. Based on this observation we indicate that this effect can lead to the low CMB multipoles suppression and gives a potential way to test Loop Quantum Cosmology models. We also consider a scenario with a post-bounce inflationary phase. The obtained power spectrum gives qualitative explanation of the CMB spectra, including low multipoles suppression. This result is a challenge to construct a consistent bounce+inflation model in the Loop Quantum Cosmology."

    DSR as an explanation of cosmological structure
    Joao Magueijo
    (Submitted on 11 Jul 2008)

    "Deformed special relativity (DSR) is one of the possible realizations of a varying speed of light (VSL). It deforms the usual quadratic dispersion relations so that the speed of light becomes energy dependent, with preferred frames avoided by postulating a non-linear representation of the Lorentz group. The theory may be used to induce a varying speed of sound capable of generating (near) scale-invariant density fluctuations, as discussed in a recent Letter. We identify the non-linear representation of the Lorentz group that leads to scale-invariance, finding a universal result. We also examine the higher order field theory that could be set up to represent it."

    Effective theory for the cosmological generation of structure
    Martin Bojowald, Aureliano Skirzewski
    8 pages, 1 figure
    (Submitted on 5 Aug 2008)

    "The current understanding of structure formation in the early universe is mainly built on a magnification of quantum fluctuations in an initial vacuum state during an early phase of accelerated universe expansion. One usually describes this process by solving equations for a quantum state of matter on a given expanding background space-time, followed by decoherence arguments for the emergence of classical inhomogeneities from the quantum fluctuations. Here, we formulate the coupling of quantum matter fields to a dynamical gravitational background in an effective framework which allows the inclusion of back-reaction effects. It is shown how quantum fluctuations couple to classical inhomogeneities and can thus manage to generate cosmic structure in an evolving background. Several specific effects follow from a qualitative analysis of the back-reaction, including a likely reduction of the overall amplitude of power in the cosmic microwave background, the occurrence of small non-Gaussianities, and a possible suppression of power for odd modes on large scales without parity violation."

    Time delay of light signals in an energy-dependent spacetime metric
    A. F. Grillo, E. Luzio, F. Mendez
    5 pages. Physical Review D77, 104033 (2008)
    (Submitted on 16 Aug 2008)

    "In this note we review the problem of time delay of photons propagating in a spacetime with a metric that explicitly depends on the energy of the particles (Gravity-Rainbow approach). We show that corrections due to this approach -- which is closely related to DSR proposal -- produce for small redshifts (z<<1) smaller time delays than in the generic Lorentz Invariance Violating case."

    The gravitational wave background from super-inflation in Loop Quantum Cosmology
    E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
    8 pages, 3 figures
    (Submitted on 1 Oct 2008)

    "We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with [tex]n_t \approx 2[/tex]. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where [tex]n_t \approx 0[/tex]."

    Proving Inflation: A Bootstrap Approach
    Latham Boyle (CITA), Paul J. Steinhardt (Princeton)
    4 pages, 2 figures
    (Submitted on 16 Oct 2008)

    "We propose a way to test the essential idea underlying the inflationary paradigm: that the universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion."
    (This is included because the authors propose a test which could actually disfavor or even rule out the occurence of inflation, with major consequences for quantum cosmo. According to the authors, inflation scenarios are falsifiable.)

    Loop quantum cosmology and tensor perturbations in the early universe
    Gianluca Calcagni, Golam Mortuza Hossain
    12 pages. Invited contribution to the special issue of Advanced Science Letters on "Quantum gravity, Cosmology and Black Holes"
    (Submitted on 23 Oct 2008)

    "We study the tensor modes of linear metric perturbations within an effective framework of loop quantum cosmology. After a review of inverse-volume and holonomy corrections in the background equations of motion, we solve the linearized tensor modes equations and extract their spectrum. Ignoring holonomy corrections, the tensor spectrum is blue tilted in the near-Planckian superinflationary regime and may be observationally disfavoured. However, in this case background dynamics is highly nonperturbative, hence the use of standard perturbative techniques may not be very reliable. On the other hand, in the quasi-classical regime the tensor index receives a small negative quantum correction, slightly enhancing the standard red tilt in slow-roll inflation. We discuss possible interpretations of this correction, which depends on the choice of semiclassical state."
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 3, 2008 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    A recent seminar talk by Smolin related to QC test prospects:
    Possibly observable cosmological consequences of quantum gravity
    See 21 October entry here:
    or download PDF slides PDF directly with:
    and wav audio
    or aif audio

    The 21 October seminar talk was about three different research gambits. The first part describes a possible alternative to the usual inflation scenarios---a different answer the horizon puzzle and explaining the CMB fluctuation spectrum. A recurrent theme is comparing quantum gravity models with cosmological observation. The Planck observatory due to be launched in 2009...see slide #23. This part of the talk referenced an earlier paper which might be useful to have available:
    Holography and the scale-invariance of density fluctuations
    Joao Magueijo, Lee Smolin, Carlo R. Contaldi
    (Submitted on 21 Nov 2006)

    "We study a scenario for the very early universe in which there is a fast phase transition from a non-geometric, high temperature phase to a low temperature, geometric phase described by a classical solution to the Einstein equations..."

    Ashtekar will be giving the 18 November seminar talk in this series--likely to be on quantum cosmology:
    Last edited: Nov 3, 2008
  4. Nov 3, 2008 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Constraining effective quantum gravity with LISA
    Nicolas Yunes, Lee Samuel Finn
    6 pages, submitted to the LISA Symposium Conference Proceedings
    (Submitted on 2 Nov 2008)

    "All modern routes leading to a quantum theory of gravity -- i.e., perturbative quantum gravitational one-loop exact correction to the global chiral current in the standard model, string theory, and perhaps even loop quantum gravity -- require supplementing the Einstein-Hilbert action with a parity-violating Chern-Simons term. Such a term leads to amplitude-birefringent gravitational wave propagation: i.e., one (circular) polarization state amplified with propagation while the other is attenuated. The proposed Laser Interferometer Space Antenna (LISA) is capable of observing gravitational wave sources at cosmological distances, suggesting the possibility that LISA observations may place a strong bound on this manifestation of quantum gravity. Here we report on a calculation of the effect that spacetime amplitude birefringence has on the signal LISA is capable of observing from inspiraling supermassive black hole binaries at large redshift. We find that the birefringence manifests itself in the observations as an anomalous precession of the binary's orbital angular momentum as it evolves toward coalescence, whose magnitude depends on the integrated history of the Chern-Simons coupling over the worldline of radiation wavefront. We estimate that LISA could place bounds on Chern-Simons modified gravity that are several orders of magnitude stronger than the present Solar System constraints, thus providing a probe of the quantum structure of spacetime."
  5. Nov 6, 2008 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    getting a a better handle on inflation, dark energy, and dark matter will help decide on nonsingular cosmology models---so even though they are periferal I will include a small sample of links to new papers concerning those issues:

    A blueprint for detecting supersymmetric dark matter in the Galactic halo
    Authors: Volker Springel (1), Simon D.M. White (1), Carlos S. Frenk (2), Julio F. Navarro (3,4), Adrian Jenkins (2), Mark Vogelsberger (1), Jie Wang (1), Aaron Ludlow (3), Amina Helmi (5) ((1) MPA, (2) Durham, (3) UVic, (4) UMass, (5) Groningen)
    to appear in Nature, 23 pages, 8 figures
    (Submitted on 5 Sep 2008)

    "Dark matter is the dominant form of matter in the universe, but its nature is unknown. It is plausibly an elementary particle, perhaps the lightest supersymmetric partner of known particle species. In this case, annihilation of dark matter in the halo of the Milky Way should produce gamma-rays at a level which may soon be observable. Previous work has argued that the annihilation signal will be dominated by emission from very small clumps (perhaps smaller even than the Earth) which would be most easily detected where they cluster together in the dark matter halos of dwarf satellite galaxies. Here we show, using the largest ever simulation of the formation of a galactic halo, that such small-scale structure will, in fact, have a negligible impact on dark matter detectability. Rather, the dominant and likely most easily detectable signal will be produced by diffuse dark matter in the main halo of the Milky Way. If the main halo is strongly detected, then small dark matter clumps should also be visible, but may well contain no stars, thereby confirming a key prediction of the Cold Dark Matter (CDM) model."
  6. Nov 8, 2008 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It looks increasingly likely that testing quantum cosmo models will involve explaining slight departures from isotropy. Pragmatically the usual assumptions of uniformity work very well and are certainly very nearly right---but it is always possible that there is a primordial lopsidedness in density: a slight tilt and the resulting slight largescale drift. It is possible that future high-precision CMB measurement, e.g. by the Planck observatory, will discover some anomalies in the CMB. This might, for instance, challenge the prevailing inflation scenarios. Ultimately QC has to explain early universe structure and either explain inflation or come up with non-inflationary answers to the puzzles that inflation was invented to solve.

    Papers and talks like these will, I expect, prove important:

    No large-angle correlations on the non-Galactic microwave sky
    Craig J. Copi, Dragan Huterer, Dominik J. Schwarz, Glenn D. Starkman
    (Submitted on 27 Aug 2008)

    We investigate the angular two-point correlation function of temperature in the WMAP maps. Updating and extending earlier results, we confirm the lack of correlations outside the Galaxy on angular scales greater than about 60 degrees at a level that would occur in 0.025 per cent of realizations of the concordance model... We argue that unless there is some ... error ...the data point towards a violation of statistical isotropy. The near-vanishing of the large-angle correlations in the cut-sky maps, together with their disagreement with results inferred from full-sky maps, remain open problems, and are very difficult to understand within the concordance model.

    Huterer and Starkman are well known. A talk presenting this paper's results with much additional explanation is:

    If the CMB is right, it is inconsistent with standard inflationary Lambda CDM
    Glenn Starkman - Case Western Reserve University
    video of talk given 30 October 2008 at Perimeter Institute

    "The Cosmic Microwave Background Radiation is our most important source of information about the early universe. Many of its features are in good agreement with the predictions of the so-called standard model of cosmology -- the Lambda Cold Dark Matter Inflationary Big Bang. However, the large-angle correlations in the microwave background exhibit several statistically significant anomalies compared to the predictions of the standard model. ... when we look at the part of the sky that we most trust -- the part outside the galactic plane, there is a dramatic lack of large angle correlations. So much so that no choice of angular powerspectrum can explain it... [using] Gaussian random statistically isotropic variables of zero mean."

    A tool for confirming the paper's results with new observations is described in:
    Testable polarization predictions for models of CMB isotropy anomalies
    Cora Dvorkin (U. Chicago), Hiranya V. Peiris (U. Chicago/Cambridge), Wayne Hu (U. Chicago)
    17 pages, 15 figures; Phys. Rev. D 77, 063008 (2008)
    (Submitted on 15 Nov 2007)

    "Anomalies in the large-scale CMB temperature sky measured by WMAP have been suggested as possible evidence for a violation of statistical isotropy on large scales. In any physical model for broken isotropy, there are testable consequences for the CMB polarization field. We develop simulation tools for predicting the polarization field in models that break statistical isotropy locally through a modulation field. ... As predictions of anomaly models, polarization statistics move beyond the a posteriori inferences that currently dominate the field."

    Wayne Hu is well known. H.V. Peiris, a Huterer PhD, is currently postdoc at Cambridge:
    http://www.ast.cam.ac.uk/~hiranya/ [Broken]
    Last edited by a moderator: May 3, 2017
  7. Nov 11, 2008 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    This paper shows where Bojowald and his co-workers are going. Their effort is focused on deriving observable consequences of Loop Quantum Cosmology. LQC is the approach where quantizing the cosmo model typically gets rid of the singularity and replaces it with a bounce. The bounce is not something intentionally contrived, but is one of several mathematical consequences of quantizing gravity in this context. The aim here is to make those LQC models falsifiable, by deriving consequences that one can then test by observation.

    Gauge invariant cosmological perturbation equations with corrections from loop quantum gravity
    Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, S. Shankaranarayanan
    (Submitted on 10 Nov 2008)

    "A consistent implementation of quantum gravity is expected to change the familiar notions of space, time and the propagation of matter in drastic ways. This will have consequences on very small scales, but also gives rise to correction terms in evolution equations of modes relevant for observations. In particular, the evolution of inhomogeneities in the very early universe should be affected. In this paper consistent evolution equations for gauge-invariant perturbations in the presence of inverse triad corrections of loop quantum gravity are derived. Some immediate effects are pointed out, for instance concerning conservation of power on large scales and non-adiabaticity. It is also emphasized that several critical corrections can only be seen to arise in a fully consistent treatment where the gauge freedom of canonical gravity is not fixed before implementing quantum corrections. In particular, metric modes must be allowed to be inhomogeneous: it is not consistent to assume only matter inhomogeneities on a quantum corrected homogeneous background geometry. In this way, stringent consistency conditions arise for possible quantization ambiguities which will eventually be further constrained observationally."

    Dark Energy, Inflation and Extra Dimensions
    Paul J. Steinhardt, Daniel Wesley
    26 pages, 1 figure
    (Submitted on 11 Nov 2008)

    "We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation-of-state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in the compact dimensions and vary with time in precise synchrony with the matter and energy density in the non-compact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications."

    Steinhardt is a world-class theoretical physicist/cosmologist. He suspected early on, around 10 years ago, that inflation might be incompatible with string's compact dimensions, so one helpful thing he did (with Neil Turok) was invent the clashing braneworld scenario that makes the world safe for string because it does away with the need for inflation. However his cyclic or ekpyrotic brane-clash cosmology has not proven so popular. Now he seems to be coming to grips with the issue. Here he has inflation challenge extra dimensions. You take your choice, can't have both, according to this analysis, unless you're willing to take on a whole lot more complication. Ways may be imagined to get around this impasse, but it's interesting and suggestive.

    I'll repeat this one because it partners with what I just listed. It shows where Steinhardt is going and how serious he is about it.
    Proving Inflation: A Bootstrap Approach
    Latham Boyle (CITA), Paul J. Steinhardt (Princeton)
    4 pages, 2 figures
    (Submitted on 16 Oct 2008)

    "We propose a way to test the essential idea underlying the inflationary paradigm: that the universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion."

    A number of inflation scenario-builders seem to just like dreaming up different inflation fantasies without getting down to the question of how we can ever be sure one or another scenario actually happened. Steinhardt is tackling this difficult problem. He proposes a way that the generic features of all inflation stories might be falsified observationally.

    Or conversely inflation might survive the proposed program of tests, lending it greater believability.
    Last edited: Nov 11, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook