arXiv subjects

Anthony Aguirre

Publications and source records attributed to Anthony Aguirre.

At least 55 records · Page 3Linked to original sources

arXiv2008

Dark Stars: Dark Matter in the First Stars leads to a New Phase of Stellar Evolution

The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the the universe. This talk presents the story of these Dark Stars. We make predictions that the first stars are very massive ($\sim 800 M_\odot$), cool (6000 K), bright ($\sim 10^6 L_\odot$), long-lived ($\sim 10^6$ years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial DM fuel runs out and fusion sets in, DM annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again.

astro-ph↗

arXiv2008

Towards observable signatures of other bubble universes II: Exact solutions for thin-wall bubble collisions

We assess the effects of a collision between two vacuum bubbles in the thin-wall limit. After describing the outcome of a generic collision possessing the expected hyperbolic symmetry, we focus on collisions experienced by a bubble containing positive vacuum energy, which could in principle contain our observable universe. We provide criteria governing whether the post-collision domain wall accelerates towards or away from this "observation" bubble, and discuss the implications for observers located at various positions inside of the bubble. Then, we identify the class of solutions which have minimal impact on the interior of the observation bubble, and derive a simple formula for the energy density of a shell of radiation emitted from such a collision. In the context of a universe undergoing false vacuum eternal inflation, these solutions are perhaps the most promising candidates for collisions that could exist within our past light cone, and therefore in principle be observable.

hep-th↗

arXiv2008

Metallicity of the intergalactic medium using pixel statistics: IV. Oxygen

We have studied the abundance of oxygen in the IGM by analyzing OVI, CIV, SiIV, and HI pixel optical depths derived from a set of high-quality VLT and Keck spectra of 17 QSOs at 2.1 < z < 3.6. Comparing OVI and CIV optical depth ratios to those in realistic, synthetic spectra drawn from a hydrodynamical simulation and comparing to existing constraints on [Si/C] places strong constraints on the ultraviolet background (UVB) model using weak priors on allowed values of [Si/O]: for example, a quasar-only background yields [Si/O] ~ 1.4, highly inconsistent with the [Si/O] ~ 0 expected from nucleosynthetic yields and with observations of metal-poor stars. Assuming a fiducial quasar+galaxy UVB consistent with these constraints yields a primary result that [O/C] = 0.66 +/- 0.06 +/- 0.2; this result pertains to gas with overdensity >~ 0.2. Consistent results are obtained by similarly comparing OVI to HI or OVI to SiIV optical depth ratios to simulation values, and also by directly ionization-correcting OVI optical depths as function of HI optical depths into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-density samples are inconsistent, suggesting either density-dependence of [O/C] or -- more likely -- prevalence of collisionally-ionized gas at high density.

astro-ph↗

arXiv2007

Eternal Inflation, past and future

Cosmological inflation, if it occurred, radically alters the picture of the `big bang', which would merely point to reheating at the end of inflation. Moreover, this reheating may be only local, so that inflation continues elsewhere and forever, continually spawning big-bang-like regions. This chapter reviews this idea of `eternal inflation', then focuses on what this may mean for the ultimate beginning of the universe. In particular, I will argue that given eternal inflation, the universe may be free of a cosmological initial singularity, might be eternal (and eternally inflating) to the past, and might obey an interesting sort of cosmological time-symmetry.

hep-th↗

arXiv2007

Towards observable signatures of other bubble universes

We evaluate the possibility of observable effects arising from collisions between vacuum bubbles in a universe undergoing false-vacuum eternal inflation. Contrary to conventional wisdom, we find that under certain assumptions most positions inside a bubble should have access to a large number of collision events. We calculate the expected number and angular size distribution of such collisions on an observer's "sky," finding that for typical observers the distribution is anisotropic and includes many bubbles, each of which will affect the majority of the observer's sky. After a qualitative discussion of the physics involved in collisions between arbitrary bubbles, we evaluate the implications of our results, and outline possible detectable effects. In an optimistic sense, then, the present paper constitutes a first step in an assessment of the possible effects of other bubble universes on the cosmic microwave background and other observables.

hep-th↗

arXiv2007

The missing metals problem. III How many metals are expelled from galaxies?

[Abridged] We revisit the metal budget at z~2. In the first two papers of this series, we already showed that ~30% (to <60% if extrapolating the LF) of the metals are observed in all z~2.5 galaxies detected in current surveys. Here, we extend our analysis to the metals outside galaxies, i.e. in intergalactic medium (IGM), using observational data and analytical calculations. Our results for the two are strikingly similar: (1) Observationally, we find that, besides the small (5%) contribution of DLAs, the forest and sub-DLAs contribute subtantially to make <30--45% of the metal budget, but neither of these appear to be sufficient to close the metal budget. The forest accounts for 15--30% depending on the UV background, and sub-DLAs for >2% to <17% depending on the ionization fraction. Together, the `missing metals' problem is substantially eased. (2) We perform analytical calculations based on the effective yield--mass relation. At z=2, we find that the method predicts that 2$--50% of the metals have been ejected from galaxies into the IGM, consistent with the observations. The metal ejection is predominantly by L<1/3L_B^*(z=2) galaxies, which are responsible for 90% the metal enrichment, while the 50 percentile is at L~1/10L^*_B(z=2). As a consequence, if indeed 50% of the metals have been ejected from galaxies, 3--5 bursts of star formation are required per galaxy prior to z=2. The ratio between the mass of metals outside galaxies to those in stars has changed from z=2 to z=0: it was 2:1 or 1:1 and is now 1:8 or 1:9. This evolution implies that a significant fraction of the IGM metals will cool and fall back into galaxies.

astro-ph↗

arXiv2007

Measures on transitions for cosmology from eternal inflation

We argue that in the context of eternal inflation in the landscape, making predictions for cosmological -- and possibly particle physics -- observables requires a measure on the possible cosmological histories as opposed to one on the vacua themselves. If significant slow-roll inflation occurs, the observables are generally determined by the history after the last transition between metastable vacua. Hence we start from several existing measures for counting vacua and develop measures for counting the transitions between vacua.

hep-th↗

arXiv2007

Hurdles for Recent Measures in Eternal Inflation

In recent literature on eternal inflation, a number of measures have been introduced which attempt to assign probabilities to different pocket universes by counting the number of each type of pocket according to a specific procedure. We give an overview of the existing measures, pointing out some interesting connections and generic predictions. For example, pairs of vacua that undergo fast transitions between themselves will be strongly favored. The resultant implications for making predictions in a generic potential landscape are discussed. We also raise a number of issues concerning the types of transitions that observers in eternal inflation are able to experience.

hep-th↗

arXiv2006

How Did the IGM Become Enriched?

The enrichment of the intergalactic medium with heavy elements is a process that lies at the nexus of poorly-understood aspects of physical cosmology. We review current understanding of the processes that may remove metals from galaxies, the basic predictions of these models, the key observational constraints on enrichment, and how intergalactic enrichment may be used to test cosmological simulations.

astro-ph↗

arXiv2006

Two Tunnels to Inflation

We investigate the formation via tunneling of inflating (false-vacuum) bubbles in a true-vacuum background, and the reverse process. Using effective potentials from the junction condition formalism, all true- and false-vacuum bubble solutions with positive interior and exterior cosmological constant, and arbitrary mass are catalogued. We find that tunneling through the same effective potential appears to describe two distinct processes: one in which the initial and final states are separated by a wormhole (the Farhi-Guth-Guven mechanism), and one in which they are either in the same hubble volume or separated by a cosmological horizon. In the zero-mass limit, the first process corresponds to the creation of an inhomogenous universe from nothing, while the second mechanism is equivalent to the nucleation of true- or false-vacuum Coleman-De Luccia bubbles. We compute the probabilities of both mechanisms in the WKB approximation using semi-classical Hamiltonian methods, and find that -- assuming both process are allowed -- neither mechanism dominates in all regimes.

gr-qc↗

arXiv2006

Dimensionless constants, cosmology and other dark matters

We identify 31 dimensionless physical constants required by particle physics and cosmology, and emphasize that both microphysical constraints and selection effects might help elucidate their origin. Axion cosmology provides an instructive example, in which these two kinds of arguments must both be taken into account, and work well together. If a Peccei-Quinn phase transition occurred before or during inflation, then the axion dark matter density will vary from place to place with a probability distribution. By calculating the net dark matter halo formation rate as a function of all four relevant cosmological parameters and assessing other constraints, we find that this probability distribution, computed at stable solar systems, is arguably peaked near the observed dark matter density. If cosmologically relevant WIMP dark matter is discovered, then one naturally expects comparable densities of WIMPs and axions, making it important to follow up with precision measurements to determine whether WIMPs account for all of the dark matter or merely part of it.

astro-ph↗

arXiv2005

Dynamics and instability of false vacuum bubbles

This paper examines the classical dynamics of false vacuum regions embedded in surrounding regions of true vacuum, in the thin-wall limit. The dynamics of all generally relativistically allowed solutions -- most but not all of which have been previously studied -- are derived, enumerated, and interpreted. We comment on the relation of these solutions to possible mechanisms whereby inflating regions may be spawned from non-inflating ones. We then calculate the dynamics of first order deviations from spherical symmetry, finding that many solutions are unstable to such aspherical perturbations. The parameter space in which the perturbations on bound solutions inevitably become nonlinear is mapped. This instability has consequences for the Farhi-Guth-Guven mechanism for baby universe production via quantum tunneling.

gr-qc↗

arXiv2005

The Spatial Distribution of Metals in the Intergalactic Medium

We investigate the impact of environment on the metallicity of the diffuse intergalactic medium. We use pixel correlation techniques to search for weak CIV and OVI absorption in the spectrum of quasar Q1422+231 in regions of the spectrum close to and far from galaxies at z ~ 3. This is achieved both by using the positions of observed Lyman break galaxies and by using strong CIV absorption as a proxy for the presence of galaxies near the line of sight. We find that the metal line absorption is a strong function of not only the HI optical depth (and thus gas density) but also proximity to highly enriched regions (and so proximity to galaxies). The parameter ``proximity to galaxies'' can account for some, but not all, of the scatter in the strength of CIV absorption for fixed HI. Finally, we find that even if we limit our analysis to the two thirds of the pixels that are at least 600 km/s from any CIV line that is strong enough to detect unambiguously (tau_CIV > 0.1), our statistical analysis reveals only slightly less CIV for fixed HI than when we analyze the whole spectrum. We conclude that while the metallicity is enhanced in regions close to (Lyman-break) galaxies, the enrichment is likely to be much more widespread than their immediate surroundings.

astro-ph↗

arXiv2005

Abundances in the high-redshift Intergalactic Medium

The enrichment of the intergalactic medium (IGM) with heavy elements provides us with a record of past star formation and with an opportunity to study the interactions between galaxies and their environments. We summarize current data analysis methods and observational constraints on abundances in the diffuse, high-redshift (z > 2) IGM. This review is targeted at interested outsiders and attempts to answer the following questions: Why should you care? What do we want to measure? How do we do it? What do we know? What are the common misconceptions?

astro-ph↗

arXiv2005

On making predictions in a multiverse: conundrums, dangers, and coincidences

The notion that there are many "universes" with different properties is one answer to the question of "why is the universe so hospitable to life?" This notion also naturally follows from current ideas in eternal inflation and string/M theory. But how do we test such a "multiverse" theory: which of the many universes do we compare to ours? This paper enumerates would would seem to be essential ingredients for making testable predictions, outlines different strategies one might take within this framework, then discusses some of the difficulties and dangers inherent in these approaches. Finally, I address the issue of whether there may be some general, qualitative predictions that multiverse theories might share.

astro-ph↗

arXiv2005

Observational Tests of Intergalactic Enrichment Models

We summarize recent results assessing the carbon and silicon abundances of the intergalactic medium (IGM) using the `pixel optical depth' technique. We briefly discuss the implications of these results for models of intergalactic enrichment, focusing on distinguishing `early' z >> 4 enrichment by the first generations of stars and objects from `late' enrichment by 2 < z < 5 Ly-break galaxies. We then discuss the comparison of observed QSO spectra to simulated spectra generated from cosmological simulations that self-consistently include enrichment, and draw qualitative implications for the general picture of intergalactic enrichment at z > 2.

astro-ph↗

arXiv2005

Confronting cosmological simulations with observations of intergalactic metals

Using the statistics of pixel optical depths, we compare HI, CIV and CIII absorption in a set of six high quality z ~ 3-4 quasar absorption spectra to that in spectra drawn from two different state-of-the-art cosmological simulations that include galactic outflows. We find that the simulations predict far too little CIV absorption unless the UVB is extremely soft, and always predict far too small CIII/CIV ratios. We note, however, that much of the enriched gas is in a phase (T ~ 10^5-10^7 K, overdensity ~ 0.1-10, Z >~0.1 Z_sol) that should cool by metal line emission -- which was not included in our simulations. When the effect of cooling is modeled, the predicted CIV absorption increases substantially, but the CIII/CIV ratios are still far too small because the density of the enriched gas is too low. Finally, we find that the predicted metal distribution is much too inhomogeneous to reproduce the observed probability distribution of CIV absorption. These findings suggest that strong z <~ 6 winds cannot fully explain the observed enrichment, and that an additional (perhaps higher-z) contribution is required.

astro-ph↗

arXiv2004

Multiple universes, cosmic coincidences, and other dark matters

Even when completely and consistently formulated, a fundamental theory of physics and cosmological boundary conditions may not give unambiguous and unique predictions for the universe we observe; indeed inflation, string/M theory, and quantum cosmology all arguably suggest that we can observe only one member of an ensemble with diverse properties. How, then, can such theories be tested? It has been variously asserted that in a future measurement we should observe the a priori most probable set of predicted properties (the ``bottom-up'' approach), or the most probable set compatible with all current observations (the ``top-down'' approach), or the most probable set consistent with the existence of observers (the ``anthropic'' approach). These inhabit a spectrum of levels of conditionalization and can lead to qualitatively different predictions. For example, in a context in which the densities of various species of dark matter vary among members of an ensemble of otherwise similar regions, from the top-down or anthropic viewpoints -- but not the bottom-up -- it would be natural for us to observe multiple types of dark matter with similar contributions to the observed dark matter density. In the anthropic approach it is also possible in principle to strengthen this argument and the limit the number of likely dark matter sub-components. In both cases the argument may be extendible to dark energy or primordial density perturbations. This implies that the anthropic approach to cosmology, introduced in part to explain "coincidences" between unrelated constituents of our universe, predicts that more, as-yet-unobserved coincidences should come to light.

hep-th↗