arXiv subjects

Michael S. Turner

Publications and source records attributed to Michael S. Turner.

At least 37 records · Page 2Linked to original sources

arXiv2003

Is Cosmic Speed-Up Due to New Gravitational Physics?

We show that cosmic acceleration can arise due to very tiny corrections to the usual gravitational action of General Relativity of the form $R^n$, with $n<0$. This eliminates the need for dark energy, though it does not address the cosmological constant problem. Since a modification to the Einstein-Hilbert action of the form $R^n$, with $n>0$, can lead to early-time inflation, our proposal provides a unified and purely gravitational origin for the early and late time accelerating phases of the Universe.

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arXiv2003

Probing Dark Energy with Supernovae: Exploiting Complementarity with the Cosmic Microwave Background

A primary goal for cosmology and particle physics over the coming decade will be to unravel the nature of the dark energy that drives the accelerated expansion of the Universe. In particular, determination of the equation-of-state of dark energy, w equivalent p/rho, and its time variation, dw/dz, will be critical for developing theoretical understanding of the new physics behind this phenomenon. Type Ia supernovae (SNe) and cosmic microwave background (CMB) anisotropy are each sensitive to the dark energy equation-of-state. SNe alone can determine w(z) with some precision, while CMB anisotropy alone cannot because of a strong degeneracy between the matter density Omega_M and w. However, we show that the Planck CMB mission can significantly improve the power of a deep SNe survey to probe w and especially dw/dz. Because CMB constraints are nearly orthogonal to SNe constraints in the Omega_M-w plane, for constraining w(z) Planck is more useful than precise determination of Omega_M. We discuss how the CMB/SNe complementarity impacts strategies for the redshift distribution of a supernova survey to determine w(z) and conclude that a well-designed sample should include a substantial number of supernovae out to redshifts z ~ 2.

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arXiv2003

Dark Energy as a Modification of the Friedmann Equation

Dark energy could actually be the manifestation of a modification to the Friedmann equation arising from new physics (e.g., extra dimensions). Writing the correction as $(1-Ω_M)H^α/H_0^{α-2}$, we explore the phenomenology and detectability of such. We show that: (i) $α$ must be $\la 1$; (ii) such a correction behaves like dark energy with equation-of-state $w_{\rm eff} = -1 + {α\over 2}$ in the recent past ($10^4> z\gg 1$) and $w=-1$ in the distant future and can mimic $w<-1$ without violating the weak-energy condition; (iii) $w_{\rm eff}$ changes, $dz/dw|_{z\sim 0.5} \sim {\cal O}(0.2)$, which is likely detectable; and (iv) a future supernova experiment like SNAP that can determine $w$ with precision $σ_w$, could determine $α$ to precision $σ_α\approx 2 σ_w$.

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arXiv2002

The New Cosmology: Mid-term Report Card for Inflation

Inflation has been the driving idea in cosmology for two decades and is a pillar of the New Cosmology. The inflationary paradigm has now passed its first round of significant tests, with two of its three basics predictions confirmed at about the 10% level. The Inflationary Paradigm has some of the truth. Over the next decade the precision of these tests, most of which involve measurements of CMB anisotropy and polarization, will improve 30 fold or more(!), testing inflation more sharply and possibly elucidating the underlying cause. Especially important in this regard is detecting the inflation-produced gravitational waves, either directly or through their CMB polarization signature. While inflation has by no means been verified, its successes have raised the bar for competitor theories: Any alternative must feature the two hallmarks of inflation: superluminal expansion and entropy production.

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arXiv2002

Dark Matter and Dark Energy: The Critical Questions

Stars account for only about 0.5% of the content of the Universe; the bulk of the Universe is optically dark. The dark side of the Universe is comprised of: at least 0.1% light neutrinos; 3.5% +/- 1% baryons; 29% +/- 4% cold dark matter; and 66% +/- 6% dark energy. Now that we have characterized the dark side of the Universe, the challenge is to understand it. The critical questions are: (1) What form do the dark baryons take? (2) What is (are) the constituent(s) of the cold dark matter? (3) What is the nature of the mysterious dark energy that is causing the Universe to speed up.

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arXiv2002

The Case for Omega_M = 0.33 +/- 0.035

For decades, the determination of the mean density of matter(Omega_M) has been tied to the distribution of light. This has led to a ``bias,'' perhaps as large as a factor of 2, in determining a key cosmological parameter. Recent measurements of the physical properties of clusters, cosmic microwave background (CMB) anisotropy and the power spectrum of mass inhomogeneity now allow a determination of Omega_M without ``visual bias.'' The early data lead to a consistent picture of the matter and baryon densities, with Omega_B = 0.039 +/- 0.0075 and Omega_M = 0.33 +/- 0.035.

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arXiv2002

Experimental Constraints on the Axion Dark Matter Halo Density

Most of the mass of the Milky Way galaxy is contributed by its halo, presumably in the form of non-interacting cold dark matter. The axion is a compelling cold dark matter candidate. We report results from a search which probes the local galactic halo axion density using the Sikivie RF cavity technique. Candidates over the frequency range 550$$ $\le$ f $\le$$$ 810 MHz (2.3$$ $μ$eV $\le$$$ m$_{a}$ $\le$ 3.4 $μ$eV) were investigated. The absence of a signal suggests that KSVZ axions contribute no more than 0.45 GeV/cm$^3$ of mass density to the local dark matter halo over this mass range.

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arXiv2002

How Cold Dark Matter Theory Explains Milgrom's Law

Milgrom noticed the remarkable fact that the gravitational effect of dark matter in galaxies only becomes important where accelerations are less than about 10^{-8} cm s^{-2} ~ cH_0. This forms the basis for his Modified Newtonian Dynamics (MOND), an alternative to particle dark matter. However, any successful theory of galactic dynamics must account for Milgrom's Law. We show how Milgrom's Law comes about in the Cold Dark Matter (CDM) theory of structure formation.

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arXiv2002

The New Cosmology

Over the past three years we have determined the basic features of our Universe. It is spatially flat; accelerating; comprised of 1/3 a new form of matter, 2/3 a new form of energy, with some ordinary matter and a dash of massive neutrinos; and it apparently began from a great burst of expansion (inflation) during which quantum noise was stretched to astrophysical size seeding cosmic structure. This ``New Cosmology'' greatly extends the highly successful hot big-bang model. Now we have to make sense of it. What is the dark matter particle? What is the nature of the dark energy? Why this mixture? How did the matter -- antimatter asymmetry arise? What is the underlying cause of inflation (if it indeed occurred)?

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arXiv2002

Making Sense Of The New Cosmology

Over the past three years we have determined the basic features of the Universe -- spatially flat; accelerating; comprised of 1/3 a new form of matter, 2/3 a new form of energy, with some ordinary matter and a dash of massive neutrinos; and apparently born from a burst of rapid expansion during which quantum noise was stretched to astrophysical size seeding cosmic structure. The New Cosmology greatly extends the highly successful hot big-bang model. Now we have to make sense of all this: What is the dark matter particle? What is the nature of the dark energy? Why this mixture? How did the matter -- antimatter asymmetry arise? What is the underlying cause of inflation (if it indeed occurred)?

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arXiv2002

A Cosmic Perspective from Lapland in 2001

A convergence of ideas, observations and technology have led to the greatest period of cosmological discovery yet. Over the past three years we have determined the basic features of our Universe. We are now challenged to make sense of what we have found. The outcome of planned experiments and observations as well as new ideas will be required. If we succeed, ours truly will be a Golden Age of Cosmology.

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arXiv2001

Dark Energy and the New Cosmology

A successor to the standard hot big-bang cosmology is emerging. It greatly extends the highly successful hot big-bang model. A key element of the New Standard Cosmology is dark energy, the causative agent for accelerated expansion. Dark energy is just possibly the most important problem in all of physics. The only laboratory up to the task of studying dark energy is the Universe itself.

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arXiv2001

Time at the Beginning

Age consistency for the Universe today has been an important cosmological test. Even more powerful consistency tests at times as early as 10^{-32} sec lie ahead in the precision era of cosmology. I outline tests based upon cosmic microwave background (CMB) anisotropy, big-bang nucleosynthesis (BBN), particle dark matter, phase transitions, and inflation. The ultimate cosmic timescale -- the fate of the Universe -- will be in doubt until the mystery of the dark energy is unraveled.

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arXiv2001

Do SNe Ia Provide Direct Evidence for Past Deceleration of the Universe?

Observations of SN 1997ff at z ~ 1.7 favor the accelerating Universe interpretation of the high-redshift type Ia supernova data over simple models of intergalactic dust or SN luminosity evolution. Taken at face-value, they provide direct lines of evidence that the Universe was decelerating in the past, an expected but untested feature of the current cosmological model. We show that the strength of this conclusion depends upon the nature of the dark energy causing the present acceleration. Only for a cosmological constant is the SNe evidence definitive. Using a new test which is independent of the contents of the Universe, we show that the SN data favor recent acceleration (z < 0.5) and past deceleration (z > 0.5).

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arXiv2001

Big-Bang Nucleosynthesis Predictions for Precision Cosmology

The determination of the primeval deuterium abundance has opened a precision era in big-bang nucleosynthesis (BBN), making accurate predictions more important than ever before. We present in analytic form new, more precise predictions for the light-element abundances and their error matrix. Using our predictions and the primeval deuterium abundance we infer a baryon density of Omega_B h^2 = 0.020 pm 0.002 (95% cl) and find no evidence for stellar production (or destruction) of 3He beyond burning D to 3He. Conclusions about 4He and 7Li currently hinge upon possible systematic error in their measurements.

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arXiv2001

New HST spectra indicate the QSO PG1718+4807 will not give the primordial deuterium abundance

The z ~ 0.701 absorption system towards the QSO PG1718+4807 is the only example of a QSO absorption system which might have a deuterium/hydrogen ratio approximately ten times the value found towards other QSOs. We have obtained new STIS spectra from the Hubble Space Telescope of the Lyman alpha and Lyman limit regions of the system. These spectra give the redshift and velocity dispersion of the neutral hydrogen which produces most of the observed absorption. The Lyman alpha line is too narrow to account for all of the observed absorption. It was previously known that extra absorption is needed on the blue side of the main H I near to the expected position of deuterium. The current data suggests with a 98% confidence level that the extra absorption is not deuterium. Some uncertainty persists because we have a low signal to noise ratio and the extra absorption - be it deuterium or hydrogen - is heavily blended with the Lyman alpha absorption from the main hydrogen component.

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arXiv2001

Constraining the Properties of Dark Energy

The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3 of the critical density, is very smoothly distributed, and has large negative pressure. Its nature is very much unknown. Most of its discernible consequences follow from its effect on evolution of the expansion rate of the Universe, which in turn affects the growth of density perturbations and the age of the Universe, and can be probed by the classical kinematic cosmological tests. Absent a compelling theoretical model (or even a class of models), we describe dark energy by an effective equation of state w=p_X/rho_X which is allowed to vary with time. We describe and compare different approaches for determining w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies and clusters, and CMB anisotropy, focusing particular attention on the use of a sample of several thousand type Ia supernova with redshifts z < 1.7, as might be gathered by the proposed SNAP satellite. Among other things, we derive optimal strategies for constraining cosmological parameters using type Ia supernovae. While in the near term CMB anisotropy will provide the first measurements of w, supernovae and number counts appear to have the most potential to probe dark energy.

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arXiv2001

A Sober Assessment of Cosmology at the New Millennium

Cosmology is the dot com of the sciences. Boom or bust. It is about nothing less than the origin and evolution of the Universe, the all of everything. It is the boldest of enterprises and not for the fainthearted. Cosmologists are the flyboys of astrophysics, and they often live up to all that image conjures up. It is hardly surprising then that words written about cosmology are rarely balanced. They verve to one extreme or the other. .... Once or twice, I have probably been guilty of irrational exuberance when it comes to cosmology.

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