arXiv subjects

Craig J. Hogan

Publications and source records attributed to Craig J. Hogan.

At least 37 records · Page 2Linked to original sources

arXiv2007

Spacetime Indeterminacy and Holographic Noise

A new kind of quantum indeterminacy of transverse position is shown to arise from quantum degrees of freedom of spacetime, based on the assumption that classical trajectories can be defined no better than the diffraction limit of Planck scale waves. Indeterminacy of the angular orientation of particle trajectories due to wave/particle duality at the Planck scale leads to indeterminacy of a nearly-flat spacetime metric, described as a small nonvanishing quantum commutation relation between transverse position operators at different events along a null trajectory. An independent derivation of the same effect is presented based on the requirement of unitarity in black hole evaporation. The indeterminacy is interpreted as a universal holographic quantum spacetime noise, with a frequency-independent spectrum of metric perturbation amplitude, h_H^2^{1/2} \simeq \sqrt{l_P}=2.3 \times 10^{-22} /\sqrt{Hz}, where l_P denotes the Planck length. The effect is estimated to be directly measurable using current interferometer technology similar to LIGO and LISA.

gr-qc↗

arXiv2007

The New Science of Gravitational Waves

A brief survey is presented of new science that will emerge during the decades ahead from direct detection of gravitational radiation. Interferometers on earth and in space will probe the universe in an entirely new way by directly sensing motions of distant matter over a range of more than a million in frequency. The most powerful sources of gravitational (or indeed any form of) energy in the universe are inspiralling and merging binary black holes; with LISA data, they will become the most distant, most completely and precisely modeled, and most accurately measured systems in astronomy outside the solar system. Other sources range from already known and named nearby Galactic binary stars, to compact objects being swallowed by massive black holes, to possible effects of new physics: phase transitions and superstrings from the early universe, or holographic noise from quantum fluctuations of local spacetime.

astro-ph↗

arXiv2007

Quantum Gravitational Uncertainty of Transverse Position

It is argued that holographic bounds on the information content of spacetime might be directly measurable. A new uncertainty principle is conjectured to arise from quantum indeterminacy of nearly flat spacetime: Angular orientations of null trajectories of spatial length L are uncertain, with standard deviation in each transverse direction Δθ> \sqrt{l_P/L}, where l_p denotes the Planck length. It is shown that this angular uncertainty corresponds to the information loss and nonlocality that occur if 3+1-D spacetime has a holographic dual description in terms of Planck-scale waves on a 2+1D screen with encoding close to the Planck diffraction limit, and agrees with covariant holographic entropy bounds on total number of degrees of freedom. The spectrum and spatial structure of predicted quantum-gravitational ``holographic noise'' are estimated to be directly measurable over a broad range of frequencies using interferometers with current technology.

astro-ph↗

arXiv2007

Holographic Indeterminacy, Uncertainty and Noise

A theory is developed to describe the nonlocal effect of spacetime quantization on position measurements transverse to macroscopic separations. Spacetime quantum states close to a classical null trajectory are approximated by plane wavefunctions of Planck wavelength (l_P) reference beams; these are used to connect transverse position operators at macroscopically separated events. Transverse positions of events with null spacetime separation, but separated by macroscopic spatial distance $L$, are shown to be quantum conjugate observables, leading to holographic indeterminacy and a new uncertainty principle, a lower bound on the standard deviation of relative transverse position Δx_\perp > \sqrt{l_PL} or angular orientation Δθ> \sqrt{l_P/L}. The resulting limit on the number of independent degrees of freedom is shown to agree quantitatively with holographic covariant entropy bounds derived from black hole physics and string theory. The theory predicts a universal ``holographic noise'' of spacetime, appearing as shear perturbations with a frequency-independent power spectral density S_H=l_P/c, or in equivalent metric perturbation units, h_{H,rms} \sqrt{l_P/c} = 2.3 \times 10^{-22} /\sqrt{Hz}. If this description of holographic phenomenology is valid, interferometers with current technology could undertake direct quantitative studies of quantum gravity.

astro-ph↗

arXiv2007

Stochastic Gravitational Wave Background from Light Cosmic Strings

Spectra of the stochastic gravitational wave backgrounds from cosmic strings are calculated and compared with present and future experimental limits. Background spectra are calculated numerically for dimensionless string tensions G mu/c^2 between 10^{-7} and 10^{-18}, and initial loop sizes as a fraction of the Hubble radius, alpha, from 0.1 to 10^{-6}. The spectra of the cosmic string backgrounds are compared with current millisecond pulsar limits and LISA sensitivity curves. For models with large stable loops (alpha=0.1), current pulsar-timing limits exclude G mu/c^2>10^{-9} and within the range of current models based on brane inflation. LISA may detect a background from strings as light as G mu/c^2 10^{-16}, corresponding to field-theory strings formed at roughly 10^{11} GeV.

astro-ph↗

arXiv2007

White Noise from Dark Matter: 21 cm Observations of Early Baryon Collapse

In concordance cosmology, dark matter density perturbations generated by inflation lead to nonlinear, virialized minihalos, into which baryons collapse at redshift $z \sim 20$. We survey here novel baryon evolution produced by a modification of the power spectrum from white noise density perturbations at scales below $k \sim 10 h {Mpc}^{-1}$ (the smallest scales currently measured with the Lyman-$α$ forest). Exotic dark matter dynamics, such as would arise from scalar dark matter with a late phase transition (similar to an axion, but with lower mass), create such an amplification of small scale power. The dark matter produced in such a phase transition collapses into minihalos, with a size given by the dark matter mass within the horizon at the phase transition. If the mass of the initial minihalos is larger than $\sim 10^{-3} M_\odot$, the modified power spectrum is found to cause widespread baryon collapse earlier than standard $Λ$CDM, leading to earlier gas heating. It also results in higher spin temperature of the baryons in the 21 cm line relative to $Λ$CDM at redshifts $z > 20$ if the mass of the minihalo is larger than $1 M_\odot$. It is estimated that experiments probing 21 cm radiation at high redshift will contribute a significant constraint on dark matter models of this type for initial minihalos larger than $\sim 10 M_\odot$. Early experiments reaching to $z\approx 15$ will constrain minihalos down to $\sim 10^3 M_\odot$.

astro-ph↗

arXiv2007

Astrophysical Effects of Scalar Dark Matter Miniclusters

We model the formation, evolution and astrophysical effects of dark compact Scalar Miniclusters (``ScaMs''). These objects arise when a scalar field, with an axion-like or Higgs-like potential, undergoes a second order phase transition below the QCD scale. Such a scalar field may couple too weakly to the standard model to be detectable directly through particle interactions, but may still be detectable by gravitational effects, such as lensing and baryon accretion by large, gravitationally bound miniclusters. The masses of these objects are shown to be constrained by the Ly$α$ power spectrum to be less than $\sim 10^4 M_\odot$, but they may be as light as classical axion miniclusters, of the order of $10^{-12} M_\odot$. We simulate the formation and nonlinear gravitational collapse of these objects around matter-radiation equality using an N-body code, estimate their gravitational lensing properties, and assess the feasibility of studying them using current and future lensing experiments. Future MACHO-type variability surveys of many background sources can reveal either high-amplification, strong lensing events, or measure density profiles directly via weak-lensing variability, depending on ScaM parameters and survey depth. However, ScaMs, due to their low internal densities, are unlikely to be responsible for apparent MACHO events already detected in the Galactic halo. A simple estimate is made of parameters that would give rise to early structure formation; in principle, early stellar collapse could be triggered by ScaMs as early as recombination, and significantly affect cosmic reionization.

astro-ph↗

arXiv2006

Nuclear Astrophysics of Worlds in the String Landscape

Motivated by landscape models in string theory, cosmic nuclear evolution is analyzed allowing the Standard Model Higgs expectation value w to take values different from that in our world (w=1), while holding the Yukawa couplings fixed. Thresholds are estimated, and astrophysical consequences are described, for several sensitive dependences of nuclear behavior on w. The dependence of the neutron-proton mass difference on w is estimated based on recent calculations of strong isospin symmetry breaking, and is used to derive the threshold of neutron-stable worlds, w ~ 0.6+/- 0.2. The effect of a stable neutron on nuclear evolution in the Big Bang and stars is shown to lead to radical differences from our world, such as a predominance of heavy r-process and s-process nuclei and a lack of normal galaxies, stars and planets. Rough estimates are reviewed of w thresholds for deuteron stability and the pp and pep reactions dominant in many stars. A simple model of nuclear resonances is used to estimate the w dependence of overall carbon and oxygen production during normal stellar nucleosynthesis; carbon production is estimated to change by a fraction ~15(1-w). Radical changes in astrophysical behavior seem to require changes in w of more than a few percent, even for the most sensitive phenomena.

astro-ph↗

arXiv2006

Gravitational Wave Sources from New Physics

Forthcoming advances in direct gravitational wave detection from kilohertz to nanohertz frequencies have unique capabilities to detect signatures from or set meaningful constraints on a wide range of new cosmological phenomena and new fundamental physics. A brief survey is presented of the post-inflationary gravitational radiation backgrounds predicted in cosmologies that include intense new classical sources such as first-order phase transitions, late-ending inflation, and dynamically active mesoscopic extra dimensions. LISA will provide the most sensitive direct probes of such phenomena near TeV energies or Terascale. LISA will also deeply probe the broadband background, and possibly bursts, from loops of cosmic superstrings predicted to form in current models of brane inflation.

astro-ph↗

arXiv2006

Gravitational Waves from Light Cosmic Strings: Backgrounds and Bursts with Large Loops

The mean spectrum and burst statistics of gravitational waves produced by a cosmological population of cosmic string loops are estimated using analytic approximations, calibrated with earlier simulations. Formulas are derived showing the dependence of observables on the string tension, in the regime where newly-formed loops are relatively large, not very much smaller than the horizon. Large loops form earlier, are more abundant, and generate a more intense stochastic background and more frequent bursts than assumed in earlier background estimates, enabling experiments to probe lighter cosmic strings of interest to string theory. Predictions are compared with instrument noise from current and future experiments, and with confusion noise from known astrophysical gravitational wave sources such as stellar and massive black hole binaries. In these large-loop models, current data from millisecond pulsar timing already suggests that the tension is less than about $10^{-10}$, a typical value expected in strings from brane inflation. LISA will be sensitive to stochastic backgrounds created by strings as light as $Gμ\approx 10^{-15}$, at frequencies where it is limited by confusion noise of Galactic stellar populations; however, for those lightest detectable strings, bursts are rarely detectable.

astro-ph↗

arXiv2006

Hubble Imaging Excludes Cosmic String Lens

The galaxy image pair CSL-1 has been a leading candidate for a cosmic string lens. High quality imaging data from the Hubble Space Telescope presented here show that it is not a lens but a pair of galaxies. The galaxies show different orientations of their principal axes, not consistent with any lens model. We present a new direct test of the straight-string lens model, using a displaced difference of the image from itself to exclude CSL-1 at high confidence.

astro-ph↗

arXiv2005

Discrete Quantum Spectrum of Observable Correlations from Inflation

The decoherence of quantum fluctuations into classical perturbations during inflation is discussed. A simple quantum mechanical argument, using a spatial particle wavefunction rather than a field description, shows that observable correlations from inflation must have a discrete spectrum, since they originate and freeze into the metric within a compact region. The number of discrete modes is estimated using a holographic bound on the number of degrees of freedom. The discreteness may be detectable in some models; for example, if there is a fundamental universal frequency spectrum, the inflationary gravitational wave background may be resolvable into discrete emission lines.

astro-ph↗

arXiv2005

Holographic Bound on Information in Inflationary Perturbations

The formation of frozen classical perturbations from vacuum quantum fluctuations during inflation is described as a unitary quantum process with apparent "decoherence" caused by the expanding spacetime. It is argued that the maximum observable information content per comoving volume in classical modes is subject to the covariant entropy bound at the time those modes decohere, leading to a new quantitative bound on the information contained in frozen field modes in phase space. This bound implies holographic correlations of large-scale cosmological perturbations that may be observable.

astro-ph↗

arXiv2004

Quarks, Electrons, and Atoms in Closely Related Universes

In a model where a multiverse wavefunction explores a multitude of vacua with different symmetries and parameters, properties of universes closely related to ours can be understood by examining the consequences of small departures of physical parameters from their observed values. The masses of the light fermions that make up the stable matter of which we are made-- the up and down quarks, and the electron-- have values in a narrow window that both allows a variety of nuclei other than protons to exist, and at the same time allows atoms with stable shells of electrons that are not devoured by their nuclei. These fundamental parameters of the Standard Model are good candidates for quantities whose values are determined through selection effects within a multiverse, since a living world of molecules needs stable nuclei other than just protons and neutrons. If the fermion masses are fixed by brane condensation or compactification of extra dimensions, there may be observable fossils of the branching event, such as a gravitational wave background.

astro-ph↗

arXiv2004

Discrete Spectrum of Inflationary Fluctuations

It is conjectured that inflation, taking account of quantum gravity, leads to a discrete spectrum of cosmological perturbations, instead of the continuous Gaussian spectrum predicted by standard field theory in an unquantized background. Heuristic models of discrete spectra are discussed, based on an inflaton mode with self gravity, a lattice of amplitude states, an entangled ensemble of modes, and the holographic or covariant entropy bound. Estimates are given for the discreteness observable in cosmic background anisotropy, galaxy clustering, and gravitational wave backgrounds.

astro-ph↗

arXiv2004

A Strategy for Finding Near Earth Objects with the SDSS Telescope

We present a detailed observational strategy for finding Near Earth Objects (NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3, which affects the sky coverage rate and the limiting apparent magnitude. We present results from 1 month, 3 year and 10 year simulations of such surveys. For each cadence and binning mode, we evaluate the possibility of achieving the Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for an albedo of 0.1). We find that an unbinned survey is most effective at detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the Spaceguard Goal after only seven years of operation. As the proposed large survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an SDSS NEO survey could make a significant contribution to the detection and photometric characterization of the NEO population.

astro-ph↗

arXiv2003

Imaging and Demography of the Host Galaxies of High-Redshift Type Ia Supernovae

We present the results of a study of the host galaxies of high redshift Type Ia supernovae (SNe Ia). We provide a catalog of 18 hosts of SNe Ia observed with the Hubble Space Telescope (HST) by the High-z Supernova Search Team (HZT), including images, scale-lengths, measurements of integrated (Hubble equivalent) BVRIZ photometry in bands where the galaxies are brighter than m ~ 25 mag, and galactocentric distances of the supernovae. We compare the residuals of SN Ia distance measurements from cosmological fits to measurable properties of the supernova host galaxies that might be expected to correlate with variable properties of the progenitor population, such as host galaxy color and position of the supernova. We find mostly null results; the current data are generally consistent with no correlations of the distance residuals with host galaxy properties in the redshift range 0.42 < z < 1.06. Although a subsample of SN hosts shows a formally significant (3-sigma) correlation between apparent V-R host color and distance residuals, the correlation is not consistent with the null results from other host colors probed by our largest samples. There is also evidence for the same correlations between SN Ia properties and host type at low redshift and high redshift. These similarities support the current practice of extrapolating properties of the nearby population to high redshifts pending more robust detections of any correlations between distance residuals from cosmological fits and host properties.

astro-ph↗

arXiv2002

Information from the Beginning

Requiring black hole evaporation to be quantum-mechanically coherent imposes a universal, finite ``holographic bound'', conjectured to be due to fundamental discreteness of quantized gravity, on the amount of information carried by any physical system. This bound is applied to the information budget in the standard slow-roll model of cosmic inflation. A simple estimate suggests that when quantum gravity is included, fluctuations during inflation have a discrete spectrum with a limited information content, only about 10^5 bits per mode, fixed by the inverse scalar perturbation amplitude. This scarcity of information may allow direct observation of quantum-gravity eigenmodes in the anisotropy of cosmic background radiation.

astro-ph↗