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Ruth A. Daly

Publications and source records attributed to Ruth A. Daly.

At least 37 records · Page 2Linked to original sources
arXiv2007

Improved Constraints on the Acceleration History of the Universe and the Properties of the Dark Energy

We extend and apply a model-independent analysis method developed earlier by Daly & Djorgovski to new samples of supernova standard candles, radio galaxy and cluster standard rulers, and use it to constrain physical properties of the dark energy as functions of redshift. Similar results are obtained for the radio galaxy and supernova data sets. The first and second derivatives of the distance are compared directly with predictions in a standard model based on General Relativity. The good agreement indicates that General Relativity provides an accurate description of the data on look-back time scales of about ten billion years. The first and second derivatives are combined to obtain the acceleration parameter, assuming only the validity of the Robertson-Walker metric, independent of a theory of gravity and of the physical nature of the dark energy. The acceleration of the universe at the current epoch is indicated by the analysis. The effect of non-zero space curvature on q(z) is explored. We solve for the pressure, energy density, equation of state, and potential and kinetic energy of the dark energy as functions of redshift assuming that General Relativity is the correct theory of gravity, and the results indicate that a cosmological constant in a spatially flat universe provides a good description of each of these quantities over the redshift range from zero to about one. We define a new function, the dark energy indicator, in terms of the first and second derivatives of the coordinate distance and show how this can be used to measure deviations of w from -1 and to obtain a new and independent measure of Omega.

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arXiv2006

A Nearly Model-Independent Characterization of Dark Energy Properties as a Function of Redshift

Understanding the acceleration of the universe and its cause is one of the key problems in physics and cosmology today, and is best studied using a variety of mutually complementary approaches. Daly and Djorgovski (2003, 2004) proposed a model independent approach to determine the expansion and acceleration history of the universe and a number of important physical parameters of the dark energy as functions of redshift directly from the data. Here, we apply the method to explicitly determine the first and second derivatives of the coordinate distance with respect to redshift and combine them to solve for the acceleration of the universe and the kinetic and potential energy density of the dark energy as functions of redshift. A data set of 228 supernova and 20 radio galaxy measurements with redshifts from zero to 1.79 is used for this study. The values we obtain are shown to be consistent with the values expected in a standard Lambda Cold Dark Matter model.

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arXiv2005

Constraints on the Physical Parameters of the Dark Energy Using a Model-Independent Approach

Understanding the physical nature of the dark energy which appears to drive the accelerated expansion of the unvierse is one of the key problems in physics and cosmology today. This important problem is best studied using a variety of mutually complementary approaches. Daly and Djorgovski (2003, 2004) proposed a model independent approach to determine a number of important physical parameters of the dark energy as functions of redshift directly from the data. Here, we expand this method to include the determinations of its potential and kinetic energy as functions of redshift. We show that the dark energy potential and kinetic energy may be written as combinations of the first and second derivatives of the coordinate distance with respect to redshift. We expand the data set to include new supernova measurements, and now use a total of 248 coordinate distances that span the redshift range from zero to 1.79. First and second derivatives of the coordinate distance are obtained as functions of redshift, and these are combined to determine the potential and kinetic energy of the dark energy as functions of redshift. An update on the redshift behavior of the dimensionless expansion rate E(z), the acceleration rate q(z), and the dark energy pressure p(z), energy density f(z), and equation of state w(z) is also presented. We find that the standard Omega = 0.3 and Lambda = 0.7 model is in an excellent agreement with the data. We also show tentative evidence that the Cardassian and Chaplygin gas models in a spatially flat universe do not fit the data as well.

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arXiv2004

Direct Constraints on the Properties and Evolution of Dark Energy

We describe a method to derive the expansion and acceleration rates directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. If one also specifies a theory of gravity we can also determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. We then apply this methodology on a modern data set of distances to Supernovae and to radio galaxies. We find that the universe transitions from deceleration to acceleration at a redshift of about 0.4, and the present value of deceleration parameter is -0.35 +/- 0.15. The standard ``concordance model'' provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. Adopting General Relativity as the theory of gravity, we obtain the redshift trends for the pressure, energy density, and equation of state of the dark energy out to a redshift of about one. They are generally consistent with the concordance model, at least out to a redshift of about 0.5, but the existing data preclude any stronger conclusions at this point.

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arXiv2004

Direct Determination of the Kinematics of the Universe and Properties of the Dark Energy as Functions of Redshift

Understanding of the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. In an earlier paper [Daly & Djorgovski (2003)] we proposed a novel method to determine the expansion rate and the deceleration parameter in a largely model-independent way, directly from the data on coordinate distances. Here we expand this methodology to include measurements of the pressure of dark energy, its normalized energy density, and the equation of state parameter as functions of redshift. We then apply this methodology to a new, combined data set of distances to supernovae and radio galaxies.

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arXiv2004

Direct Determinations of the Redshift Behavior of the Pressure, Energy Density, and Equation of State of the Dark Energy and the Acceleration of the Universe

One of the goals of current cosmological studies is the determination of the expansion and acceleration rates of the universe as functions of redshift, and the determination of the properties of the dark energy that can explain these observations. Here the expansion and acceleration rates are determined directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. We use the latest set of distances to SN standard candles from Riess et al. (2004), supplemented by data on radio galaxy standard ruler sizes, as described by Daly and Djorgovski (2003, 2004). We find that the universe transitions from acceleration to deceleration at a redshift of about 0.4. The standard "concordance model" provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. The expansion and acceleration rates are then combined with a theory of gravity to determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. Adopting General Relativity as the correct theory of gravity, the redshift trends for the pressure, energy density, and equation of state of the dark energy out to redshifts of about one are determined, and are found to be generally consistent with the concordance model.

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arXiv2003

A Model-Independent Determination of the Expansion and Acceleration Rates of the Universe as a Function of Redshift and Constraints on Dark Energy

Determination of the expansion and acceleration history of the universe is one of the fundamental goals of cosmology. Detailed measurements of these rates as a function of redshift can provide new physical insights into the nature and evolution of the dark energy, which apparently dominates the global dynamics of the universe at the present epoch. We present here dimensionless coordinate distances y(z) to twenty radio galaxies reaching out to z of about 1.8, the redshift range currently not covered by Supernova standard candle observations. We develop a simple numerical method for a direct determination of the expansion and acceleration rates, E(z) and q(z), from the data, which makes no assumptions about the underlying cosmological model or the equation of state parameter w. This differential method is in contrast to the traditional cosmological tests, where particular model equations are integrated and then compared with the observations. The new model-independent method is applied to currently available Supernova data and the data on radio galaxies presented here. We derive the expansion rate of the universe as a function of redshift, E(z), and for the first time obtain a direct estimate of the acceleration rate of the universe as a function of redshift, q(z), in a way that is independent of assumptions regarding the dark energy and its redshift evolution. The current observations indicate that the universe transitions from acceleration to deceleration at a redshift greater than 0.3, with a best fit estimate of about 0.45; this transition redshift and our determinations of E(z) are broadly in agreement with the currently popular Friedmann-Lemaitre cosmology with Omega = 0.3, and Lambda = 0.7, even though no model assumptions are made in deriving the fits for E(z) and q(z).

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arXiv2002

Radio Galaxies and the Acceleration of the Universe Beyond a Redshift of One

Radio galaxies provide a means to determine the coordinate distance,the luminosity distance, the dimensionless luminosity distance, or the angular size distance to sources with redshifts as large as two. Dimensionless coodinate distances for 55 supernovae and 20 radio galaxies are presented and discussed here. The radio galaxy results are consistent with those obtained using supernovae, suggesting that neither method is plagued by unknown systematic errors. The acceleration parameter q(z) and the expansion rate H(z) or dimensionless expansion rate E(z) can be determined directly from the data without having to make assumptions regarding the nature or evolution of the ``dark energy.'' The expansion rate E(z) can be determined from the first derivative of the dimensionless coordinate distance, and the acceleration parameter can be determined from a combination of the first and second derivatives of the dimensionless coordinate distance. A model-independent determination of E(z) will allow the properties and redshift evolution of the ``dark energy'' to be determined, and a model-independent determination of q(z) will allow the redshift at which the universe transitions from acceleration to deceleration to be determined directly. Determination of E(z) and q(z) may also elucidate possible systematic errors in the determinations of the dimensionless coordinate distances.

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arXiv2002

Constraints on the Cluster Environments and Hot Spot Magnetic Field Strengths of the Radio Sources 3C254 and 3C280

We present new Chandra Observatory observations with archival HST and radio observations of 3C254, a radio quasara at z=0.734, and 3C280, a radio galaxy at z=0.996. We report the detection of X-ray and possible HST optical counterparts to the radio hot spots in 3C280 and of an X-ray counterpart to the radio hot spot in 3C254. We present constraints on the presence of X-ray clusters and on the magnetic field strengths in and around the radio hot spots. The spatial resolution of Chandra allows us to show that these sources are not in hot, massive clusters. The extended emission seen in ROSAT observations is resolved into point sources. The IGM around these sources is demonstrably not dense and hot. We conclude that radio sources are not reliable signposts of massive clusters at moderate redshifts. X-ray synchrotron emission could explain the radio, optical, and X-ray hot spot fluxes in 3C280, but it would require continuous acceleration of electrons to high Lorentz factors, since the synchrotron lifetime required to produce the X-ray emission is of order a human lifetime. SSC with or without IC scattering of the CMB can also explain the X-ray emission, but not the optical. We review all of the physical mechanisms and summarize our current constraints on the magnetic field strengths in andaround the hot spots of 3C254 and 3C280.

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arXiv2002

Quintessence, Cosmology, and Fanaroff-Riley Type IIB Radio Galaxies

Fanaroff-Riley type IIb (FR IIb) radio galaxies provide a modified standard yardstick that allows constraints to be placed on global cosmological parameters. A sample of 20 FR IIb radio galaxies with redshifts between 0 and 2 are compared with the parent population of 70 radio galaxies to determine the coordinate distance to each source. The coordinate-distance determinations are used to constrain the current mean mass-energy density of quintessence $Ω_q$, the equation of state of the quintessence w, and the current mean mass-energy density of nonrelativistic matter $Ω_m$; zero space curvature is assumed. Radio galaxies alone indicate that the universe is currently accelerating in its expansion (with 84% confidence); most of the allowed parameter space falls within the accelerating universe region on the $Ω_m$-w plane. This provides verification of the acceleration of the universe indicated by high-redshift supernovae and suggests that neither method is plagued by systematic errors. It is found that $Ω_m$ must be less than about 0.5 and the equation of state w of the quintessence must lie between -0.25 and -2.5 at about 90% confidence. Fits of the radio galaxy data constrain the model parameter $β$, which describes a relation between the beam power of the active galactic nucleus (AGN) and the total energy expelled through large-scale jets. It is shown that the empirically determined model parameter is consistent with models in which the outflow results from the electromagnetic extraction of rotational energy from the central compact object. A specific relation between the strength of the magnetic field near the AGN and the spin angular momentum per unit mass of the central compact object is predicted.

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arXiv2002

High-Redshift Radio Galaxies as a Cosmological Tool: Exploration of a Key Assumption and Comparison with Supernova Results

There are many different approaches to using observations to constrain or determine the global cosmological parameters that describe our universe. Methods that rely upon a determination of the coordinate distance to high-redshift sources are particularly useful because they do not involve assumptions about the clustering properties of matter, or the evolution of this clustering. Two of the methods currently being used to determine the coordinate distance to high-redshift sources are the radio galaxy method and the supernova method. These methods are similar in their dependence on the coordinate distance. Here, the radio galaxy method is briefly described and results are presented. One of the underlying assumptions of the method is explored. In addition, the method is compared and contrasted to the supernova method. The constraints imposed on global cosmological parameters by radio galaxies are consistent with those imposed by supernovae. For a universe that is spatially flat with mean mass density $Ω_m$ in non-relativistic matter and mean mass density 1- $Ω_m$ in quintessence, radio galaxies alone indicate at 84 % confidence that the expansion of the universe is accelerating at the current epoch. And, independent of whether or not the universe is spatially flat, radio galaxies alone indicate at 95 % confidence that $Ω_m$ must be less than 0.6 at the current epoch.

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arXiv2001

Cosmological Parameters and Quintessence From Radio Galaxies

FRIIb radio galaxies provide a tool to determine the coordinate distance to sources out to redshifts of two. The coordinate distance depends on the present values of global cosmological parameters, quintessence, and the equation of state of quintessence, and provides one of the cleanest determinations of global cosmological parameters because it does not depend on the clustering properties of any mass-energy components. The modified standard candle method (using type Ia supernove) of determining the coordinate distance is compared in detail with the modified standard yardstick method (using FRIIb radio galaxies); the methods are found to be complementary. The most significant difference between the methods is that the radio galaxy method is completely independent of the local distance scale and the properties of local sources, while the supernovae method is very closely tied to the local distance scale and the properties of local sources. FRIIb radio galaxies provide one of the very few reliable probes of the coordinate distance to sources with redshifts out to two. The method indicates that the current value of the density parameter in non-relativistic matter must be low irrespective of whether the universe is spatially flat, and of whether a significant cosmological constant or quintessence pervades the universe at the present epoch. FRIIb radio galaxies indicate that the universe is currently accelerating in its expansion if the primary components of the universe at the present epoch are non-relativistic matter and quintessence, and the universe is spatially flat.

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arXiv2000

Global Cosmological Parameters Determined Using Classical Double Radio Galaxies

A sample of 20 powerful extended radio galaxies with redshifts between zero and two were used to determine constraints on global cosmological parameters. Data for six radio sources were obtained from the VLA archive, analyzed, and combined with the sample of 14 radio galaxies used previously by Guerra & Daly to determine cosmological parameters. The results are consistent with our previous results, and indicate that the current value of the mean mass density of the universe is significantly less than the critical value. A universe with $Ω_m$ of unity is ruled out at 99.0% confidence, and the best fitting values of $Ω_m$ in matter are $0.10^{+0.25}_{-0.10}$ and $-0.25^{+0.35}_{-0.25}$ assuming zero space curvature and zero cosmological constant, respectively. Note that identical results obtain when the low redshift bin, which includes Cygnus A, is excluded; these results are independent of whether the radio source Cygnus A is included. The method does not rely on a zero-redshift normalization. The radio properties of each source are also used to determine the density of the gas in the vicinity of the source, and the beam power of the source. The six new radio sources have physical characteristics similar to those found for the original 14 sources. The density of the gas around these radio sources is typical of gas in present day clusters of galaxies. The beam powers are typically about $10^{45} \hbox{erg s}^{-1}$.

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arXiv2000

Determinations of Key Physical Parameters Related to Classical Double Radio Sources

Multi-frequency radio observations of the radio bridge of a powerful classical double radio source can be used to determine: the beam power of the jets emanating from the AGN; the total time the source will actively produce jets that power large-scale radio emission; the thermal pressure of the medium in the vicinity of the radio source; and the total mass, including dark matter, of the galaxy or cluster of galaxies traced by the ambient gas that surrounds the radio source. Empirical determinations of each of these quantities are obtained and analyzed for 22 radio sources. Typical beam powers are about $10^{45} {erg s}^{-1}$. The characteristic or total time the AGN will actively produce a collimated outflow is estimated. Typical total lifetimes are $\sim (10^7$ to $10^8$) years. Total masses, and mass-density profiles, similar to those of low-redshift clusters of galaxies are obtained. Thus, some clusters of galaxies, or cores of clusters, exist at redshifts of one to two. A new method of estimating the thermal pressure of the ambient gas in the vicinity of a powerful classical double radio source is presented. This new estimate is independent of synchrotron and inverse Compton aging arguments, and depends only upon the properties of the radio lobe and the shape of the radio bridge. Thermal pressures on the order of $10^{-10} {dyne cm}^{-2}$, typical of gas in low-redshift clusters of galaxies, are found for the environments of the sources studied here. It is shown that appreciable amounts of cosmic microwave background diminution (the Sunyaev-Zel'dovich effect) are expected from many of these clusters. This could be detected at high frequency where the emission from the radio sources is weak.

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arXiv1998

Global Cosmological Parameters Measured Using Classical Double Radio Sources

Fourteen classical double radio galaxies with redshifts between zero and two were used to determine the cosmological parameters $Ω_m$, $Ω_Λ$, and $Ω_k$, where these are the normalized values of the mean mass density, cosmological constant, and space curvature at the present epoch. A low value of $Ω_m$ is obtained, and $Ω_m = 1$ is ruled out with 97.5% confidence. The low value of $Ω_m$ determined using the radio source method described here is also indicated by several independent tests. Thus, it appears that either a cosmological constant, or space curvature, is significant at the present epoch. This means that the universe is undergoing, or has recently undergone, a transition away from a state of matter domination and into a state where either a cosmological constant or space curvature is determining the expansion rate of the universe. The low value of $Ω_m$ presented here and by Guerra & Daly (1998) means that we can state with 97.5 % confidence that the universe will continue to expand forever.

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arXiv1997

Central Engines of AGN: Properties of Collimated Outflows and Applications for Cosmology

Powerful extended radio galaxies in the 3CR survey are observed out to redshifts of about 2. For redshifts greater than about 0.3, the average lobe-lobe size of the sources decreases monotonically with redshift for all reasonable cosmological parameter choices. This suggests that the characteristic time for which an AGN produces a highly collimated outflow that powers the radio emission is shorter for the high-redshift sources than it is for low-redshift sources. The fact that the characteristic time for which an AGN produces a collimated outflow decreases with redshift implies that the physical mechanism that gives rise to the large scale jets is not Eddington limited. Higher redshift sources have higher radio powers due the flux limit of the sample, and sources with higher radio powers have higher beam powers relative to lower redshift sources. Both the beam power and active lifetime of the AGN are controlled by processes occurring at the heart of the AGN, and may be related for non-Eddington limited processes. The relation between the active lifetime, $t_*$, and the beam power, $L_j$, of powerful extended radio galaxies is investigated here and found to be $t_*\proptoL_j^{-2/3}$. A comparison of the redshift evolution of the characteristic source sizes with that of the average lobe-lobe size for the 3CR sample of radio galaxies could be used to constrain cosmology.

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arXiv1997

Outflow Angles, Bulk Lorentz Factors, and Kinematics of Outflows from the Cores of AGN

Outflow angles and bulk Lorentz factors for 43 sources that have proper motions compiled by Vermeulen & Cohen (1994) are computed on the basis of Doppler factors and observed apparent motions in the plane of the sky. These estimates of outflow angles and bulk Lorentz factors are discussed along with their agreement with orientation unified models of AGN. Intrinsic (i.e. rest frame) brightness temperatures computed by using the inverse Compton and equipartition Doppler factor are discussed along with their relevance to the ``Inverse Compton catastrophe''. Intrinsic luminosity densities and luminosities are presented, and the role of systematic errors is discussed. These studies are carried out using a sample of 100 compact radio sources compiled by Ghisellini et al. (1993). Error estimates for previously computed inverse Compton Doppler factors and equipartition Doppler factors are presented for these sources, along with a few updates of these Doppler factor estimates.

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