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Papers by Einar Gudmundsson

Astrophys J, 2002

We consider ever-expanding big bang models with a cosmological constant, Λ, and investigate in de... more We consider ever-expanding big bang models with a cosmological constant, Λ, and investigate in detail the evolution of the observable part of the universe. We also discuss quintessence models from the same point of view. A new concept, the Λ-sphere (or Q-sphere, in the case of quintessence) is introduced. This is the surface in our visible universe that bounds the region where dark energy dominates the expansion, and within which the universe is accelerating. We follow the evolution of this surface as the universe expands, and we also investigate the evolution of the particle and event horizons as well as the Hubble surface. We calculate the extent of the observable universe and the portion of it that can be seen at different epochs. Furthermore, we trace the changes in redshift, apparent magnitude and apparent size of distant sources through cosmic history. Our approach is different from, but complementary to, most other contemporary investigations, which concentrate on the past light cone at the present epoch. When presenting numerical results we use the FRW world model with Ωm0=0.30 and ΩΛ0=0.70 as our standard cosmological model. In this model the Λ-sphere is at a redshift of 0.67, and within a few Hubble times the event horizon will be stationary at a fixed proper distance of 5.1 Gpc (assuming h0=0.7). All cosmological sources with present redshift larger than 1.7 have by now crossed the event horizon and are therefore completely out of causal contact.

The Fabulous Destiny of Galaxies Bridging Past and Present, 2006

Page 1. Conference “The Fabulous Destiny of Galaxies: Bridging Past and Present”, Marseille, June... more Page 1. Conference “The Fabulous Destiny of Galaxies: Bridging Past and Present”, Marseille, June 20-24, 2005 Host galaxies of gamma-ray bursts: A cosmological tracer of galaxy formation Stéphanie Courty, Gunnlaugur Björnsson, Einar H. Guðmundsson ...

Astron Astrophys, 2007

Aims. Use the standard fireball model to create virtual populations of gamma-ray burst afterglows... more Aims. Use the standard fireball model to create virtual populations of gamma-ray burst afterglows and study their luminosity functions. Methods. We randomly vary the parameters of the standard fireball model to create virtual populations of afterglows. We use the luminosity of each burst at an observer's time of 1 day to create a luminosity function and compare our results with available observational data to assess the internal consistency of the standard fireball model. Results. We show that the luminosity functions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but not very dependent on the model parameter distributions used to create the virtual populations. Comparison with observations shows that while there is good general agreement with the data, it is difficult to explain simultaneously the X-ray and optical data. Possible reasons for this are discussed and the most likely one is that the standard fireball model is incomplete and that decoupling of the X-ray and optical emission mechanism may be needed.

Several GRB afterglow light curves deviate strongly from the power law decay observed in most bur... more Several GRB afterglow light curves deviate strongly from the power law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavio r. As an example we consider GRB 021004 that exhibited strong light curve variations and has a reasonably

Monthly Notices of the Royal Astronomical Society, Oct 1, 1990

We apply the three classical tests of observational cosmology, i.e. the m(z), the phi(z) and the ... more We apply the three classical tests of observational cosmology, i.e. the m(z), the phi(z) and the N(z) tests, to world models with pressure and demonstrate that they are degenerate with respect to the pressure parameter alpha_0_ (= 3 P_0_/rho_0_ c^2^) in the range 0-1. This degeneracy is due to the fact that for all redshifts, z, less than ~2 the results of the tests for a given value of the deceleration parameter, qfl, are almost identical for models with different values of all, the differences being very small as compared to observational uncertainties. Only at very high redshifts (z >~5) are there appreciable differences between models with different aralpha_0_ and observationally relevant values of q_0_. These tests can therefore not be used at present to determine aii and as a result the allowed range in the density parameter {OMEGA}_0_ is q_0_ <= {OMEGA}_0_ <= 2q_0_ for a given value of q_0_ (and a cosmological constant equal to zero)i The fourth classical test, the so- called 'age of the universe test', is potentially much more powerful than the other three for determining the cosmological parameters, aalpha and {OMEGA}_0_. At present, however, age estimates are so uncertain that no definite conclusion can be drawn.

AIP Conference Proceedings, 2007

ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afte... more ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afterglows and study their luminosity distributions. We find that the luminosity distributions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but the choice of the model parameter distributions used to create the virtual populations has small effects on the results. Our results are compared to observations and indicate that a number of afterglows with low optical luminosities go unobserved.

Mon Notic Roy Astron Soc, 1995

We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers livin... more We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers living at different cosmic epochs. For this purpose we apply the three observational tests of classical cosmology to closed universe models without a cosmological constant, both for the expanding and for the contracting phases of their evolution. In particular, we investigate how the Hubble diagram and other plots of observational quantities, such as angular size and number density of galaxies, change with cosmic time. Once recollapse has started, these diagrams become multivalued for certain ranges of the observational variables. Light from nearby galaxies is blueshifted whereas galaxies further away have redshifted spectra, these two regions of the observable universe being separated by a `surface of zero redshift'. In a contracting universe two different images of the same galaxy may appear in opposite directions of the sky since observers can, in principle, see beyond their own antipoles. Because of this and the fact that the cosmological diagrams are multivalued, interpretation of cosmological data could be considerably more difficult in the contracting phase than during the expanding phase of the evolution.

Monthly Notices of the Royal Astronomical Society

ABSTRACT

The Astrophysical Journal

ABSTRACT

Astrophys J, 2002

We consider ever-expanding big bang models with a cosmological constant, Λ, and investigate in de... more We consider ever-expanding big bang models with a cosmological constant, Λ, and investigate in detail the evolution of the observable part of the universe. We also discuss quintessence models from the same point of view. A new concept, the Λ-sphere (or Q-sphere, in the case of quintessence) is introduced. This is the surface in our visible universe that bounds the region where dark energy dominates the expansion, and within which the universe is accelerating. We follow the evolution of this surface as the universe expands, and we also investigate the evolution of the particle and event horizons as well as the Hubble surface. We calculate the extent of the observable universe and the portion of it that can be seen at different epochs. Furthermore, we trace the changes in redshift, apparent magnitude and apparent size of distant sources through cosmic history. Our approach is different from, but complementary to, most other contemporary investigations, which concentrate on the past light cone at the present epoch. When presenting numerical results we use the FRW world model with Ωm0=0.30 and ΩΛ0=0.70 as our standard cosmological model. In this model the Λ-sphere is at a redshift of 0.67, and within a few Hubble times the event horizon will be stationary at a fixed proper distance of 5.1 Gpc (assuming h0=0.7). All cosmological sources with present redshift larger than 1.7 have by now crossed the event horizon and are therefore completely out of causal contact.

The Fabulous Destiny of Galaxies Bridging Past and Present, 2006

Page 1. Conference “The Fabulous Destiny of Galaxies: Bridging Past and Present”, Marseille, June... more Page 1. Conference “The Fabulous Destiny of Galaxies: Bridging Past and Present”, Marseille, June 20-24, 2005 Host galaxies of gamma-ray bursts: A cosmological tracer of galaxy formation Stéphanie Courty, Gunnlaugur Björnsson, Einar H. Guðmundsson ...

Astron Astrophys, 2007

Aims. Use the standard fireball model to create virtual populations of gamma-ray burst afterglows... more Aims. Use the standard fireball model to create virtual populations of gamma-ray burst afterglows and study their luminosity functions. Methods. We randomly vary the parameters of the standard fireball model to create virtual populations of afterglows. We use the luminosity of each burst at an observer's time of 1 day to create a luminosity function and compare our results with available observational data to assess the internal consistency of the standard fireball model. Results. We show that the luminosity functions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but not very dependent on the model parameter distributions used to create the virtual populations. Comparison with observations shows that while there is good general agreement with the data, it is difficult to explain simultaneously the X-ray and optical data. Possible reasons for this are discussed and the most likely one is that the standard fireball model is incomplete and that decoupling of the X-ray and optical emission mechanism may be needed.

Several GRB afterglow light curves deviate strongly from the power law decay observed in most bur... more Several GRB afterglow light curves deviate strongly from the power law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavio r. As an example we consider GRB 021004 that exhibited strong light curve variations and has a reasonably

Monthly Notices of the Royal Astronomical Society, Oct 1, 1990

We apply the three classical tests of observational cosmology, i.e. the m(z), the phi(z) and the ... more We apply the three classical tests of observational cosmology, i.e. the m(z), the phi(z) and the N(z) tests, to world models with pressure and demonstrate that they are degenerate with respect to the pressure parameter alpha_0_ (= 3 P_0_/rho_0_ c^2^) in the range 0-1. This degeneracy is due to the fact that for all redshifts, z, less than ~2 the results of the tests for a given value of the deceleration parameter, qfl, are almost identical for models with different values of all, the differences being very small as compared to observational uncertainties. Only at very high redshifts (z >~5) are there appreciable differences between models with different aralpha_0_ and observationally relevant values of q_0_. These tests can therefore not be used at present to determine aii and as a result the allowed range in the density parameter {OMEGA}_0_ is q_0_ <= {OMEGA}_0_ <= 2q_0_ for a given value of q_0_ (and a cosmological constant equal to zero)i The fourth classical test, the so- called 'age of the universe test', is potentially much more powerful than the other three for determining the cosmological parameters, aalpha and {OMEGA}_0_. At present, however, age estimates are so uncertain that no definite conclusion can be drawn.

AIP Conference Proceedings, 2007

ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afte... more ABSTRACT Using the standard fireball model, we create virtual populations of gamma-ray burst afterglows and study their luminosity distributions. We find that the luminosity distributions can be described by a function similar to a log normal distribution with an exponential cutoff. The function parameters are frequency dependent but the choice of the model parameter distributions used to create the virtual populations has small effects on the results. Our results are compared to observations and indicate that a number of afterglows with low optical luminosities go unobserved.

Mon Notic Roy Astron Soc, 1995

We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers livin... more We investigate how a closed Friedmann-Robertson-Walker universe would appear to astronomers living at different cosmic epochs. For this purpose we apply the three observational tests of classical cosmology to closed universe models without a cosmological constant, both for the expanding and for the contracting phases of their evolution. In particular, we investigate how the Hubble diagram and other plots of observational quantities, such as angular size and number density of galaxies, change with cosmic time. Once recollapse has started, these diagrams become multivalued for certain ranges of the observational variables. Light from nearby galaxies is blueshifted whereas galaxies further away have redshifted spectra, these two regions of the observable universe being separated by a `surface of zero redshift'. In a contracting universe two different images of the same galaxy may appear in opposite directions of the sky since observers can, in principle, see beyond their own antipoles. Because of this and the fact that the cosmological diagrams are multivalued, interpretation of cosmological data could be considerably more difficult in the contracting phase than during the expanding phase of the evolution.

Monthly Notices of the Royal Astronomical Society

ABSTRACT

The Astrophysical Journal

ABSTRACT