Gravitational Wave Sources from New Physics

TL;DR

The paper surveys post-inflationary gravitational-wave backgrounds from new physics and assesses their detectability with near-future detectors, especially LISA. It analyzes four classical GW sources—phase transitions with relativistic turbulence, dynamics of extra dimensions, late inflationary reheating, and cosmic superstrings—and maps their spectra to detector bands using current theoretical models. The findings indicate that TeV-scale phase transitions can produce detectable backgrounds in the LISA band; extra dimensions and brane dynamics offer alternative spectral patterns; late reheating could yield a loud background with energy density up to $10^{-3}$ of the thermal plasma; and cosmic superstrings can generate a nearly flat $\Omega_{GW}$ with substantial observational reach, including potential cusp bursts. Detecting these signals would probe physics beyond the Standard Model at the Terascale and illuminate otherwise inaccessible epochs of cosmic history.

Abstract

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.

Figures (4)

• Figure 1: Reshifted Hubble frequency $\dot a$ as a function of cosmic scale factor $a$ (both base-10 log scales). Horizontal lines are shown indicating observable gravitational wave frequency bands: the current Hubble frequency (CMB techniques), inverse years to decades (millisecond pulsar timing, MSP), millihertz (LISA), one Hz ( the vision mission BBO, Big Bang Observer), and kilohertz (LIGO,VIRGO,GEO). Heavy solid lines show a standard inflationary model that has expanded by about 27 orders of magnitude since GUT scale inflation. Later-ending inflationary models are also shown, reheating at $10^7$ TeV scale energy ($f\approx 10^3$ Hz,$H\approx 10^{-25.5}$ sec) and 1 TeV scale ($f\approx 10^{-4}$ Hz, $H\approx 10^{-11.5}$ sec). Shaded regions correspond to accessible cosmological sources of gravitational waves: on the right branch, classical sources such as defects, phase transitions, and brane displacement excitations; on the left branch, inflationary quantum sources. Shaded region above the main lines shows the scale factor where light cosmic superstrings (CS) emit most of their observed background energy at each frequency. Hatched regions show the range covered by currently accessible cosmological data on CMB, large scale structure (LSS) and big bang nucleosynthesis (BBN).
• Figure 2: Overview of spectra of gravitational wave backgrounds from high redshift. The energy density of gravitational waves per log frequency, in units of the critical density, is shown for inflationary quantum (tensor) fluctuations, and for turbulent cosmic phase transitions at 1 TeV and $10^6$ TeV (which peak somewhat above the Hubble frequencies at those times since the largest scale motions are subhorizon in scale). Relativistic motion close to the horizon scale can create classical backgrounds that approach within a few orders of magnitude of the CMB energy density, and are far more intense than inflationary quantum generation. Inflation is shown here for an optimistic large amplitude and flat spectrum, but this neglects effects that might make it smaller: a dashed line shows the sign of effects of damping or postinflationary modifications to the equation of state. CMB shows the possible region explored by microwave background polarization.
• Figure 3: A schematic background spectrum from a strong $\approx$TeV-scale phase transition, superimposed on a plot of LISA's sensitivity to broadband backgrounds, fromHogan:2001jn. The upper axis shows the energy scale (that is, roughly the fourth root of the mean energy density) corresponding to the redshifted Hubble frequency. The sample background shown is not calculated exactly but is typical of estimated spectra, showing how the shape of the spectrum falls off on each side of the characteristic frequency and might be used as a diagnostic of the source. The lower boundaries of the shaded regions show roughly where astrophysical backgrounds or instrument noise limit the measurements.
• Figure 4: Predicted gravitational wave backgrounds from a population of superstring loops, and a summary of foreground and instrument noise sourcesHogan:2006we. Broad band energy density is shown in units of the critical density for $h_0=1$, as a function of frequency, for a model where the typical size of newly formed loops is a fraction $\alpha=0.1$ of the horizon. Radiation from loop populations at high redshift (H) and present-day (P) is shown, labled by the value of $\log_{10}(G\mu)$. Noise levels are shown for current millisecond pulsar data (MSP), and the projected LISA sensitivity in maximum resolution and Sagnac modes. Confusion noise is shown for massive black hole binaries (MBHB), the summed Galactic binary population including binary white dwarfs (UB+WUMa+GCWDB+CV), and extragalactic populations of white dwarfs (XGCWDB) and neutron stars (XGNSB). Dotted curves show the contributions of $z>1$ loops where they are subdominant to the P contributions. Current (MSP) sensitivity is at about $G\mu\approx 10^{-10}$, and LISA will reach to around $G\mu\approx 10^{-15}$.