arXiv subjects

Robert J. Scherrer

Publications and source records attributed to Robert J. Scherrer.

At least 109 records · Page 6Linked to original sources

arXiv2005

Phantom Dark Energy, Cosmic Doomsday, and the Coincidence Problem

Phantom dark energy models, with w < -1, are characterized by a future singularity and therefore a finite lifetime for the universe. Because the future singularity is triggered by the onset of dark-energy domination, the universe spends a significant fraction of its total lifetime in a state for which the dark energy and matter densities are roughly comparable. We calculate, as a function of w, the fraction of the total lifetime of the universe for which the dark energy and matter densities differ by less than the ratio r_0 in either direction. For r_0 = 10, this fraction varies from 1/3 to 1/8 as w varies from -1.5 to -1.1; the fraction is smaller for smaller values of r_0. This result indicates that the coincidence problem is significantly ameliorated in phantom-dominated cosmologies with a future singularity.

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arXiv2004

Do Fermions and Bosons Produce the Same Gravitational Field?

We examine some cosmological consequences of gravity coupling with different strength to fermions and bosons. We show that this leads to a different perturbation of the standard picture of primordial nucleosynthesis than the addition of extra neutrino types or overall scaling of the value of G. Observed abundances of deuterium and helium-4 place bounds on the ratio of the bosonic gravitational constant (G_B) to the fermionic gravitational constant (G_F) of 0.45 < G_B/G_F < 0.92 at 1-sigma, and 0.33 < G_B/G_F < 1.10 at 2-sigma. A value of G_B < G_F can reconcile the current "tension" between the abundances of deuterium and helium-4 predicted by primordial nucleosynthesis. We comment briefly on other cosmological effects.

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arXiv2004

Purely kinetic k-essence as unified dark matter

We examine k-essence models in which the Lagrangian p is a function only of the derivatives of a scalar field phi and does not depend explicity on phi. The evolution of phi for an arbitrary functional form for p can be given in terms of an exact analytic solution. For quite general conditions on the functional form of p, such models can evolve to a state characterized by a density scaling with the scale factor as rho = rho_0 + rho_1(a/a_0)^{-3}, but with a sound speed c_s^2 << 1 at all times. Such models can serve as a unified model for dark matter and dark energy, while avoiding the problems of the generalized Chaplygin gas models, which are due to a non-negligible sound speed in these models. A dark energy component with c_s << 1 serves to suppress cosmic microwave background fluctuations on large angular scales.

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arXiv2004

Constraints on Non-Standard Recombination from Recent CMB Observations

We examine cosmic microwave background constraints on variations in the recombination history of the universe. We use a very general extension to the standard model of recombination characterized by two parameters, a and b, which multiply the overall rates of recombination and ionization and the binding energies of hydrogen respectively. We utilize WMAP temperature and TE cross-correlation data, along with temperature data at smaller scales from ACBAR and CBI, to place constraints on these parameters. The range of recombination histories which gives the best fit to the data is -0.6 < log(a) < 0.5 and 0.85 < b < 1.15, forming a diagonal region from low-a/high-b to high-a/low-b. We find z_rec = 1055 +- 25 and δz = 198 +- 5 at 68% confidence for the range of recombination models which are best able to reproduce the observed CMB power spectra. Standard recombination provides an acceptable fit to the data; while there is still room for non-standard recombination, the allowed variation is small.

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arXiv2004

The Uncertainty in Newton's Constant and Precision Predictions of the Primordial Helium Abundance

The current uncertainty in Newton's constant, G_N, is of the order of 0.15%. For values of the baryon to photon ratio consistent with both cosmic microwave background observations and the primordial deuterium abundance, this uncertainty in G_N corresponds to an uncertainty in the primordial 4He mass fraction, Y_P, of +-1.3 x 10^{-4}. This uncertainty in Y_P is comparable to the effect from the current uncertainty in the neutron lifetime, which is often treated as the dominant uncertainty in calculations of Y_P. Recent measurements of G_N seem to be converging within a smaller range; a reduction in the estimated error on G_N by a factor of 10 would essentially eliminate it as a source of uncertainty in the calculation of the primordial 4He abundance.

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arXiv2003

The Evolution of Inverse Power Law Quintessence at Low Redshift

Quintessence models based on a scalar field, phi, with an inverse power law potential display simple tracking behavior at early times, when the quintessence energy density, rho_phi, is sub-dominant. At late times, when rho_phi becomes comparable to the matter density, the evolution of phi diverges from its scaling behavior. We calculate the first order departure of phi from its tracker solution at low redshift. Our results for the evolution of phi, rho_phi, Omega_phi, and w are suprisingly accurate even down to z=0. We find that w and Omega_phi are related linearly to first order. We also derive a semi-analytic expression for w(z) which is accurate to within a few percent. Our analytic techniques are potentially applicable to any quintessence model in which the quintessence component comes to dominate at late times.

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arXiv2003

Big Bang nucleosynthesis and cosmic microwave background constraints on the time variation of the Higgs vacuum expectation value

We derive constraints on the time variation of the Higgs vacuum expectation value $<ϕ>$ through the effects on Big Bang nucleosynthesis (BBN) and the cosmic microwave background (CMB). In the former case, we include the (previously-neglected) effect of the change in the deuteron binding energy, which alters both the $^4$He and deuterium abundances significantly. We find that the current BBN limits on the relative change in \higgs are $-(0.6 - 0.7) \times 10^{-2} < Delta<ϕ>/<ϕ> < (1.5 - 2.0) \times 10^{-2}$, where the exact limits depend on the model we choose for the dependence of the deuteron binding energy on \higgs.The limits from the current CMB data are much weaker.

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arXiv2002

Big Bang Nucleosynthesis Constraints on Brane Cosmologies

We examine constraints from Big Bang nucleosynthesis on type II Randall-Sundrum brane cosmologies with both a dark radiation component and a quadratic term that depends on the 5-dimensional Planck mass, M_5. Using limits on the abundances of deuterium and helium-4, we calculate the allowed region in the M_5-dark radiation plane and derive the precise BBN bound on M_5 alone with no dark radiation: M_5 > 13 TeV.

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arXiv2002

Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy

We consider the effect of inhomogeneous neutrino degeneracy on Big Bang nucleosynthesis for the case where the distribution of neutrino chemical potentials is given by a Gaussian. The chemical potential fluctuations are taken to be isocurvature, so that only inhomogeneities in the electron chemical potential are relevant. Then the final element abundances are a function only of the baryon-photon ratio $η$, the effective number of additional neutrinos $ΔN_ν$, the mean electron neutrino degeneracy parameter $\bar ξ$, and the rms fluctuation of the degeneracy parameter, $σ_ξ$. We find that for fixed $η$, $ΔN_ν$, and $\bar ξ$, the abundances of helium-4, deuterium, and lithium-7 are, in general, increasing functions of $σ_ξ$. Hence, the effect of adding a Gaussian distribution for the electron neutrino degeneracy parameter is to decrease the allowed range for $η$. We show that this result can be generalized to a wide variety of distributions for $ξ$.

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arXiv2002

Cosmic microwave background and large scale structure limits on the interaction between dark matter and baryons

We study the effect on the cosmic microwave background (CMB) anisotropy and large scale structure (LSS) power spectrum of a scattering interaction between cold dark matter and baryons. This scattering alters the CMB anisotropy and LSS spectrum through momentum transfer between the cold dark matter particles and the baryons. We find that current CMB observations can put an upper limit on the scattering cross section which is comparable with or slightly stronger than previous disk heating constraints at masses greater than 1 GeV, and much stronger at smaller masses. When large-scale structure constraints are added to the CMB limits, our constraint is more stringent than this previous limit at all masses. In particular, a dark matter-baryon scattering cross section comparable to the ``Spergel-Steinhardt'' cross section is ruled out for dark matter mass greater than 1 GeV.

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arXiv2002

Prospects for Determining the Equation of State of the Dark Energy: What can be Learned from Multiple Observables?

The dark energy that appears to produce the accelerating expansion of the universe can be characterized by an equation of state p=wρwith w<-1/3. A number of observational tests have been proposed to study the value or redshift dependence of w, including SN Ia distances, the Sunyaev-Zel'dovich effect, cluster abundances, strong and weak gravitational lensing, galaxy and quasar clustering, galaxy ages, the \lya forest, and CMB anisotropies. The proposed observational tests based on these phenomena measure either the distance-redshift relation d(z), the Hubble parameter H(z), the age of the universe t(z), the linear growth factor D_1(z), or some combination of these quantities. We compute the evolution of these four observables, and of the combination H(z)d(z) that enters the Alcock-Paczyznski anisotropy test, in models with constant w, in quintessence models with some simple forms of the potential V(ϕ), and in toy models that allow more radical time variations of w. Measurement of any of these quantities to precision of a few percent is generally sufficient to discriminate between w=-1 and w=-2/3. However, the time-dependence predicted in quintessence models is extremely difficult to discern because the quintessence component is dynamically unimportant at the redshifts where w departs substantially from its low-z value. Even for the toy models that allow substantial changes in w at low redshift, there is always a constant-w model that produces very similar evolution of all of the observables simultaneously. We conclude that measurement of the effective equation of state of the dark energy may be achieved by several independent routes in the next few years, but that detecting time-variation in this equation of state will prove very difficult except in specialized cases.

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arXiv2001

How Does CMB + BBN Constrain New Physics?

Recent cosmic microwave background (CMB) results from BOOMERANG, MAXIMA, and DASI provide cosmological constraints on new physics that can be competitive with those derived from Big Bang Nucleosynthesis (BBN). In particular, both CMB and BBN can be used to place limits on models involving neutrino degeneracy and additional relativistic degrees of freedom. However, for the case of the CMB, these constraints are, in general, sensitive to the assumed priors. We examine the CMB and BBN constraints on such models and study the sensitivity of ``new physics" to the assumed priors. If we add a constraint on the age of the universe ($t_0 \ga 11$ Gyr), then for models with a cosmological constant, the range of baryon densities and neutrino degeneracy parameters allowed by the CMB and BBN is fairly robust: $η_{10} = 6.0 \pm 0.6$, \deln $\la 6$, $ξ_e \la 0.3$. In the absence of new physics, models without a cosmological constant are only marginally compatible with recent CMB observations (excluded at the 93% confidence level).

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arXiv2001

The Real and Redshift Space Density Distribution Function for Large-Scale Structure in the Spherical Collapse Approximation

We use the spherical collapse (SC) approximation to derive expressions for the smoothed redshift-space probability distribution function (PDF), as well as the $p$-order hierarchical amplitudes $S_p$, in both real and redshift space. We compare our results with numerical simulations, focusing on the $Ω=1$ standard CDM model, where redshift distortions are strongest. We find good agreement between the SC predictions and the numerical PDF in real space even for $σ_L \simgt 1$, where $σ_L$ is the linearly-evolved rms fluctuation on the smoothing scale. In redshift space, reasonable agreement is possible only for $σ_L \simlt 0.4$. Numerical simulations also yield a simple empirical relation between the real-space PDF and redshift-space PDF: we find that for $σ\simlt 1$, the redshift space PDF, P[δ_z], is, to a good approximation, a simple rescaling of the real space PDF, P[δ], i.e., P[δ/σ] d[δ/σ] = P[δ_z/σ_z] d[δ_z/σ_z], where $σ$ and σ_z are the real-space and redshift-space rms fluctuations, respectively. This result applies well beyond the validity of linear perturbation theory, and it is a good fit for both the standard CDM model and the Lambda-CDM model. It breaks down for SCDM at $σ\approx 1$, but provides a good fit to the Λ-CDM models for $σ$ as large as 0.8.

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arXiv2001

Extended quintessence and the primordial helium abundance

In extended quintessence models, a scalar field which couples to the curvature scalar R provides most of the energy density of the universe. We point out that such models can also lead naturally to a decrease in the primordial abundance of helium-4, relieving the tension which currently exists between the primordial helium-4 abundance inferred from observations and the amount predicted by Standard Big Bang Nucleosynthesis (SBBN) corresponding to the observed deuterium abundance. Using negative power-law potentials for the quintessence field, we determine the range of model parameters which can lead to an interesting reduction in the helium-4 abundance, and we show that it overlaps with the region allowed by other constraints on extended quintessence models.

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arXiv2001

Recent CMB Observations and the Ionization History of the Universe

Interest in non-standard recombination scenarios has been spurred by recent cosmic microwave background (CMB) results from BOOMERANG and MAXIMA, which show an unexpectedly low second acoustic peak, resulting in a best-fit baryon density that is 50% larger than the prediction of big-bang nucleosynthesis (BBN). This apparent discrepancy can be avoided if the universe has a non-standard ionization history in which the recombination of hydrogen is significantly delayed relative to the standard model. While future CMB observations may eliminate this discrepancy, it is useful to develop a general framework for analyzing non-standard ionization histories. We develop such a framework, examining non-standard models in which the hydrogen binding energy E_b and the overall expression for the time rate of change of the ionized fraction of electrons are multiplied by arbitrary factors. This set of models includes a number of previously-proposed models as special cases. We find a wide range of models with delayed recombination that are able to fit the CMB data with a baryon density in accordance with BBN, but there are even allowed models with earlier recombination than in the standard model. A generic prediction of these models is that the third acoustic CMB peak should be very low relative to what is found in the standard model. This is the case even for the models with earlier recombination than in the standard model, because here the third peak is lowered by an increased diffusion damping at recombination relative to the standard model. Interestingly, the specific height of the third peak depends sensitively on the model parameters, so that future CMB measurements will be able to distinguish between different non-standard recombination scenarios.

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arXiv2000

Cosmic Microwave Background constraint on residual annihilations of relic particles

Energy injected into the Cosmic Microwave Background at redshifts z<10^6 will distort its spectrum permanently. In this paper we discuss the distortion caused by annihilations of relic particles. We use the observational bounds on deviations from a Planck spectrum to constrain a combination of annihilation cross section, mass, and abundance. For particles with (s-wave) annihilation cross section, <σ|v|>=σ_0, the bound is f[(σ_0/6e-27cm^3/s)(Ω_{X\bar{X}}h^2)^2]/(m_X/MeV)<0.2, where m_X is the particle mass, Ω_{X\bar{X}} is the fraction of the critical density the particle and its antiparticle contribute if they survive to the present time, h=H_0/(100km/s/Mpc), H_0 is the Hubble constant, and f is the fraction of the annihilation energy that interacts electromagnetically. We also compute the less stringent limits for p-wave annihilation. We update other bounds on residual annihilations and compare them to our CMB bound.

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arXiv2000

Self-interacting Warm Dark Matter

It has been shown by many independent studies that the cold dark matter scenario produces singular galactic dark halos, in strong contrast with observations. Possible remedies are that either the dark matter is warm so that it has significant thermal motion or that the dark matter has strong self interactions. We have combined these ideas to calculate the linear mass power spectrum and the spectrum of cosmic microwave background (CMB) fluctuations for self-interacting warm dark matter. Our results indicate that such models have more power on small scales than is the case for the standard warm dark matter model, with a CMB fluctuation spectrum which is nearly indistinguishable from standard cold dark matter. This enhanced small-scale power may provide better agreement with the observations than does standard warm dark matter.

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arXiv2000

The Effect of Time Variation in the Higgs Vacuum Expectation Value on the Cosmic Microwave Background

A time variation in the Higgs vacuum expectation value alters the electron mass and thereby changes the ionization history of the universe. This change produces a measurable imprint on the pattern of cosmic microwave background (CMB) fluctuations. The nuclear masses and nuclear binding energies, as well as the Fermi coupling constant, are also altered, with negligible impact on the CMB. We calculate the changes in the spectrum of the CMB fluctuations as a function of the change in the electron mass. We find that future CMB experiments could be sensitive to |Δm_e/m_e| \sim |ΔG_F/G_F| \sim 10^{-2} - 10^{-3}. However, we also show that a change in the electron mass is nearly, but not exactly, degenerate with a change in the fine-structure constant. If both the electron mass and the fine-structure constant are time-varying, the corresponding CMB limits are much weaker, particularly for l < 1000.

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