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Michael S. Turner

Publications and source records attributed to Michael S. Turner.

At least 73 records · Page 4Linked to original sources
arXiv1998

Cosmology Solved?

For two decades the hot big-bang model as been referred to as the standard cosmology -- and for good reason. For just as long cosmologists have known that there are fundamental questions that are not answered by the standard cosmology and point to a grander theory. The best candidate for that grander theory is inflation + cold dark matter; it can extend our understanding of the Universe back to 10^-32 sec. There is now prima facie evidence for the two basic tenets of this new paradigm: flat Universe and scale-invariant spectrum of Gaussian density perturbations, and an avalanche of telling cosmological observations is coming. If inflation + cold dark matter is correct, then there are new, fundamental questions to be answered, most notably the nature of the dark energy that seems to account for 60% of the critical density and how inflation fits into a unified theory of the forces and particles. These are exciting times in cosmology!

astro-ph↗
arXiv1998

Cosmology Solved? Maybe

For two decades the hot big-bang model as been referred to as the standard cosmology -- and for good reason. For just as long cosmologists have known that there are fundamental questions that are not answered by the standard cosmology and point to a grander theory. The best candidate for that grander theory is inflation + cold dark matter. It holds that the Universe is flat, that slowly moving elementary particles left over from the earliest moments provide the cosmic infrastructure, and that the primeval density inhomogeneities that seed all the structure arose from quantum fluctuations. There is now prima facie evidence that supports two basic tenets of this paradigm. An avalanche of high-quality cosmological observations will soon make this case stronger or will break it. Key questions remain to be answered; foremost among them are: identification and detection of the cold dark matter particles and elucidation of the dark-energy component. These are exciting times in cosmology!

astro-ph↗
arXiv1998

Cosmology Solved? Quite Possibly!

The discovery of the cosmic microwave background (CMB) in 1964 by Penzias and Wilson led to the establishment of the hot big-bang cosmological model some ten years later. Discoveries made in 1998 may ultimately have as profound an effect on our understanding of the origin and evolution of the Universe. Taken at face value, they confirm the basic tenets of Inflation + Cold Dark Matter, a bold and expansive theory that addresses all the fundamental questions left unanswered by the hot big-bang model and holds that the Universe is flat, slowly moving elementary particles provide the cosmic infrastructure, and quantum fluctuations seeded all the structure seen in the Universe today. Just as it took a decade to establish the hot big-bang model after the discovery of the CMB, it will likely take another ten years to establish the latest addition to the standard cosmology and make the answer to ``Cosmology Solved?'', ``YES!'' Whether or not 1998 proves to be a cosmic milestone, the coming avalanche of high-quality cosmological data promises to make the next twenty years an extremely exciting period for cosmology.

astro-ph↗
arXiv1998

Constraining Variations in the Fine-Structure Constant with the Cosmic Microwave Background

Any time variation in the fine-structure constant alters the ionization history of the universe and therefore changes the pattern of cosmic microwave background fluctuations. We calculate the changes in the spectrum of these fluctuations as a function of the change in the fine-structure constant, and we find that these changes are dominated by the change in the redshift of recombination due to the shift in the binding energy of hydrogen. We estimate the accuracy with which the next generation of cosmic microwave background experiments might constrain any variation in the fine-structure constant at z of roughly 1000. We find that such experiments could potentially be sensitive to $|Δα/α| \sim 10^{-2} - 10^{-3}$.

astro-ph↗
arXiv1998

Large-scale Structure from Quantum Fluctuations in the Early Universe

A better understanding of the formation of large-scale structure in the Universe is arguably the most pressing question in cosmology. The most compelling and promising theoretical paradigm, Inflation + Cold Dark Matter, holds that the density inhomogeneities that seeded the formation of structure in the Universe originated from quantum fluctuations arising during inflation and that the bulk of the dark matter exists as slowing moving elementary particles (`cold dark matter') left over from the earliest, fiery moments. Large redshift surveys (such as the SDSS and 2dF) and high-resolution measurements of CBR anisotropy (to be made by the MAP and Planck Surveyor satellites) have the potential to decisively test Inflation + Cold Dark Matter and to open a window to the very early Universe and fundamental physics.

astro-ph↗
arXiv1998

Probing Unstable Massive Neutrinos with Current Cosmic Microwave Background Observations

The pattern of anisotropies in the Cosmic Microwave Background depends upon the masses and lifetimes of the three neutrino species. A neutrino species of mass greater than 10 eV with lifetime between 10^{13} sec and 10^{17} sec leaves a very distinct signature (due to the integrated Sachs-Wolfe effect): the anisotropies at large angles are predicted to be comparable to those on degree scales. Present data exclude such a possibility and hence this region of parameter space. For $m_ν\simeq 30$ eV, $τ\simeq 10^{13}$ sec, we find an interesting possibility: the Integrated Sachs Wolfe peak produced by the decaying neutrino in low-$Ω$ models mimics the acoustic peak expected in an $Ω= 1$ model.

astro-ph↗
arXiv1998

Precision Detection of the Cosmic Neutrino Background

In the standard Big Bang cosmology the canonical value for the ratio of relic neutrinos to CMB photons is 9/11. Within the framework of the Standard Model of particle physics there are small corrections, in sum about 1%, due to slight heating of neutrinos by electron/positron annihilations and finite-temperature QED effects. We show that this leads to changes in the predicted cosmic microwave background (CMB) anisotropies that might be detected by future satellite experiments. NASA's MAP and ESA's PLANCK should be able to test the canonical prediction to a precision of 1% or better and could confirm these corrections.

astro-ph↗
arXiv1998

Prospects for measuring Inflation parameters with the CMB

We present the status of our study into the feasibility of estimating the parameters of inflation from planned satellite observations of the anisotropy of the cosmic microwave background (CMB).We describe a perturbative procedure for studying inflation models with soft departures from scale free spectra and find that in this case the parameters of inflation are likely to be well determined. We also demonstrate that radical departures from scale free initial perturbations with broad features in $k$-space can be be measured by Planck. However, allowing for very fine features in $k$-space can very strongly compromise the ability to measure cosmological parameters.

astro-ph↗
arXiv1998

Results from a High-Sensitivity Search for Cosmic Axions

We report the first results of a high-sensitivity (10^{-23} Watt) search for light halo axions through their conversion to microwave photons. At 90 percent confidence we exclude a KSVZ axion of mass 2.9 x 10^{-6} eV to 3.3 x 10^{-6} eV as the dark matter in the halo of our Galaxy.

astro-ph↗
arXiv1997

No Need for MACHOS in the Halo

A simple interpretation of the more than dozen microlensing events seen in the direction of the LMC is a halo population of MACHOs which accounts for about half of the mass of the Galaxy. Such an interpretation is not without its problems, and we show that current microlensing data can, with some advantage, be explained by dark components of the disk and spheroid, whose total mass is only about 10% of the mass of the Galaxy.

astro-ph↗
arXiv1997

Expectations for the Difference Between Local and Global Measurements of the Hubble Constant

There are irreducible differences between the Hubble constant measured locally and the global value. They are due to density perturbations and finite sample volume (cosmic variance) and finite number of objects in the sample (sampling variance). We quantify these differences for a suite of COBE-normalized CDM models that are consistent with the observed large-scale structure. For small samples of objects that only extend out to 10,000 km/sec, the variance can approach 4%. For the largest samples of Type Ia supernovae (SNeIa), which include about 40 objects and extend out to almost 40,000 km/sec, the variance is 1-2% and is dominated by sampling variance. Sampling and cosmic variance may be an important consideration in comparing local determinations of the Hubble constant with precision determinations of the global value that will be made from high-resolution maps of CBR anisotropy.

astro-ph↗
arXiv1997

Big-bang Nucleosynthesis Enters the Precision Era

The last parameter of big-bang nucleosynthesis, the baryon density, is being pinned down by measurements of the deuterium abundance in high-redshift hydrogen clouds. When it is determined, it will fix the primeval light-element abundances. D, ^3He and ^7Li will become ``tracers'' for the study of Galactic and stellar chemical evolution, and big-bang nucleosynthesis will become an even sharper probe of particle physics, e.g., the bound to the number of light neutrino species will be tightened significantly. Two key tests of the consistency of the standard theory are on the horizon: an independent, high-precision determination of the baryon density from anisotropy of the cosmic background radiation and a precision determination of the primeval $^4$He abundance.

astro-ph↗
arXiv1997

Pre-Big-Bang Inflation Requires Fine Tuning

The pre-big-bang cosmology inspired by superstring theories has been suggested as an alternative to slow-roll inflation. We analyze, in both the Jordan and Einstein frames, the effect of spatial curvature on this scenario and show that too much curvature --- of either sign --- reduces the duration of the inflationary era to such an extent that the flatness and horizon problems are not solved. Hence, a fine-tuning of initial conditions is required to obtain enough inflation to solve the cosmological problems.

hep-th↗
arXiv1997

Ribbons on the CBR Sky: A Powerful Test of a Baryon Symmetric Universe

If the Universe consists of domains of matter and antimatter, annihilations at domain interfaces leave a distinctive imprint on the Cosmic Background Radiation (CBR) sky. The signature is anisotropies in the form of long, thin ribbons of width $θ_W\sim 0.1^\circ$, separated by angle $θ_L\simeq 1^\circ(L/100h^{-1}{Mpc})$ where L is the characteristic domain size, and y-distortion parameter $y \approx 10^{-6}$. Such a pattern could potentially be detected by the high-resolution CBR anisotropy experiments planned for the next decade, and such experiments may finally settle the question of whether or not our Hubble volume is baryon symmetric.

astro-ph↗
arXiv1997

Microlensing and the Composition of the Galactic Halo

By means of extensive galactic modeling we study the implications of the more than 100 microlensing events that have now been observed for the composition of the dark halo of the Galaxy. Based on the currently published data, including the 2nd year MACHO results, the halo MACHO fraction is less than 60% in most models and the likelihood function for the halo MACHO fraction peaks around 20% - 40%, consistent with expectations for cold dark matter models.

astro-ph↗
arXiv1997

Inflationary Cosmology

Inflation is a bold and expansive extension of the Standard Cosmology. It holds the promise to extend our understanding of the Universe to within 10^{-32}sec of the big bang and answer most of the pressing questions in cosmology. Its boldest assertion is that all the structure observed in the Universe today arose from quantum-mechanical fluctuations on subatomic scales. A flood of cosmological observations and laboratory experiments are testing inflation and within five years we should know whether inflation is to become part of a new standard cosmology.

astro-ph↗
arXiv1997

Cosmology 1996

The current state of cosmology is easy to summarize: a very successful standard model -- the hot big-bang cosmology -- that accounts for the evolution of the Universe from 10^{-2} sec until the present; bold ideas based upon early-Universe physics -- foremost among them inflation and cold dark matter -- that can extend the standard cosmology to times as early as 10^{-32} sec and address the most pressing questions; and a flood of observations -- from determinations of the Hubble constant to measurements of CBR anisotropy -- that are testing inflation and cold dark matter.

astro-ph↗