arXiv subjects

Joao Magueijo

Publications and source records attributed to Joao Magueijo.

At least 127 records · Page 7Linked to original sources

arXiv2000

Can a changing $α$ explain the Supernovae results?

We show that the Supernovae results, implying evidence for an accelerating Universe, may be closely related to the recent discovery of redshift dependence in the fine structure constant $α$. The link is a class of varying speed of light (VSL) theories which contain cosmological solutions similar to quintessence. During the radiation dominated epoch the cosmological constant $Λ$ is prevented from dominating the Universe by the usual VSL mechanism. In the matter epoch the varying c effects switch off, allowing $Λ$ to eventually surface and lead to an accelerating Universe. By the time this happens the residual variations in c imply a changing $α$ at a rate that is in agreement with observations.

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arXiv2000

New non-Gaussian feature in COBE-DMR Four Year Maps

We extend a previous bispectrum analysis of the Cosmic Microwave Background temperature anisotropy, allowing for the presence of correlations between different angular scales. We find a strong non-Gaussian signal in the ``inter-scale'' components of the bispectrum: their observed values concentrate close to zero instead of displaying the scatter expected from Gaussian maps. This signal is present over the range of multipoles $\ell=6 -18$, in contrast with previous detections. We attempt to attribute this effect to galactic foreground contamination, pixelization effects, possible anomalies in the noise, documented systematic errors studied by the COBE team, and the effect of assumptions used in our Monte Carlo simulations. Within this class of systematic errors the confidence level for rejecting Gaussianity varies between 97% and 99.8%.

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arXiv2000

Cosmic defects and cosmology

We provide a pedagogical overview of defect models of structure formation. We first introduce the concept of topological defect, and describe how to classify them. We then show how defects might be produced in phase transitions in the Early Universe and approach non-pathological scaling solutions. A very heuristic account of structure formation with defects is then provided, following which we introduce the tool box required for high precision calculations of CMB and LSS power spectra in these theories. The decomposition into scalar vector and tensor modes is reviewed, and then we introduce the concept of unequal-time correlator. We use isotropy and causality to constrain the form of these correlators. We finally show how these correlators may be decomposed into eigenmodes, thereby reducing a defect problem to a series of ``inflation'' problems. We conclude with a short description of results in these theories and how they fare against observations. We finally describe yet another application of topological defects in cosmology: baryogenesis.

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arXiv2000

Cosmological parameter dependence in local string theories of structure formation

We perform a new and accurate study of the dependence on cosmological parameters of structure formation with local cosmic strings. The crucial new ingredients are the inclusion of the effects of gravitational backreaction on the evolution of the network, and the accurate evolution of the network through the radiation to matter transition. Our work re-iterates the fact that expanding Universe numerical simulations only probe a transient regime, and we incorporate our results into the unequal time correlators recently measured. We then compute the CMB and CDM fluctuations' power spectra for various values of the Hubble constant $H_0$ and baryon fraction $Ω_b$. We find that, whereas the dependence on $Ω_b$ is negligible, there is still a strong dependence on $H_0$.

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arXiv1999

Imaginative Cosmology

We review a few off-the-beaten-track ideas in cosmology. They solve a variety of fundamental problems; also they are fun. We start with a description of non-singular dilaton cosmology. In these scenarios gravity is modified so that the Universe does not have a singular birth. We then present a variety of ideas mixing string theory and cosmology. These solve the cosmological problems usually solved by inflation, and furthermore shed light upon the issue of the number of dimensions of our Universe. We finally review several aspects of the varying speed of light theory. We show how the horizon, flatness, and cosmological constant problems may be solved in this scenario. We finally present a possible experimental test for a realization of this theory: a test in which the Supernovae results are to be combined with recent evidence for redshift dependence in the fine structure constant.

hep-ph↗

arXiv1999

Early structure formation with cold plus hot dark matter --- a success of strings plus inflation model

Quantum fluctuations created during inflation can account for the observed matter distribution in the linear regime if the universe has two components of dark matter, one which is cold and collisionless, and the other which is hot and free streams on small scales. However, this free streaming property of the hot component prevents early structure formation, and since objects, such as damped Lyman-$α$ systems, have been observed at high redshift, it is necessary to produce more power on small scales. Here, we show that the situation can be improved substantially in models where cosmic strings are formed at the end of inflation, and in which both inflation and strings participate in the generation of structure.

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arXiv1999

Big Bang riddles and their revelations

We describe how cosmology has converged towards a beautiful model of the Universe: the Big Bang Universe. We praise this model, but show there is a dark side to it. This dark side is usually called ``the cosmological problems'': a set of coincidences and fine tuning features required for the Big Bang Universe to be possible. After reviewing these ``riddles'' we show how they have acted as windows into the very early Universe, revealing new physics and new cosmology just as the Universe came into being. We describe inflation, pre Big Bang, and varying speed of light theories. At the end of the millennium, these proposals are seen respectively as a paradigm, a tentative idea, and outright speculation.

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arXiv1999

The statistical physics of cosmological networks of string loops

We solve numerically the Boltzmann equation describing the evolution of a cosmic string network which contains only loops. In Minkowski space time the equilibrium solution predicted by statistical mechanics is recovered, and we prove that this solution is stable to non-linear perturbations provided that their energy does not exceed the critical energy for the Hagedorn transition. In expanding Einstein - de Sitter Universes we probe the distribution of loops with length much smaller than the horizon. For these loops we discover stable scaling solutions both in the radiation and matter dominated epochs. The shape of these solutions is very different in the two eras, with much higher energy density in the radiation epoch, and a larger average loop length in the matter epoch. These results suggest that if the conditions for formation of loop networks are indeed satisfied, these could in principle be good candidates for structure formation.

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arXiv1999

A new statistic for picking out Non-Gaussianity in the CMB

In this paper we propose a new statistic capable of detecting non-Gaussianity in the CMB. The statistic is defined in Fourier space, and therefore naturally separates angular scales. It consists of taking another Fourier transform, in angle, over the Fourier modes within a given ring of scales. Like other Fourier space statistics, our statistic outdoes more conventional methods when faced with combinations of Gaussian processes (be they noise or signal) and a non-Gaussian signal which dominates only on some scales. However, unlike previous efforts along these lines, our statistic is successful in recognizing multiple non-Gaussian patterns in a single field. We discuss various applications, in which the Gaussian component may be noise or primordial signal, and the non-Gaussian component may be a cosmic string map, or some geometrical construction mimicking, say, small scale dust maps.

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arXiv1999

Structure formation with strings plus inflation: a new paradigm

Recent developments in inflation model building, based on supersymmetry, have produced compelling models in which strings are produced at the end of inflation. In such models the cosmological perturbations are seeded both by the defects and by the quantum fluctuations. We show that such models produce qualitatively new and desirable predictions for CMB anisotropies and the CDM power spectrum. This remark should put an end to the long term animosity between defect and inflationary scenarios of structure formation.

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arXiv1999

Where is the COBE maps' non-Gaussianity?

We review our recent claim that there is evidence of non-Gaussianity in the 4 Year COBE DMR data. We present some new results concerning the effect of the galactic cut upon the non-Gaussian signal. These findings imply a localization of the non-Gaussian signal on the Northern galactic hemisphere.

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arXiv1999

Solutions to the Quasi-flatness and Quasi-lambda Problems

Big Bang models of the Universe predict rapid domination by curvature, a paradox known as the flatness problem. Solutions to this problem usually leave the Universe exactly flat for every practical purpose. Explaining a nearly but not exactly flat current Universe is a new problem, which we label the quasi-flatness problem. We show how theories incorporating time-varying coupling constants could drive the Universe to a late-time near-flat attractor. A similar problem may be posed with regards to the cosmological constant $Λ$, the quasi-lambda problem, and we exhibit a solution to this problem as well.

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arXiv1999

Solving the Flatness and Quasi-flatness Problems in Brans-Dicke Cosmologies with a Varying Light Speed

We define the flatness and quasi-flatness problems in cosmological models. We seek solutions to both problems in homogeneous and isotropic Brans-Dicke cosmologies with varying speed of light. We formulate this theory and find perturbative, non-perturbative, and asymptotic solutions using both numerical and analytical methods. For a particular range of variations of the speed of light the flatness problem can be solved. Under other conditions there exists a late-time attractor with a constant value of Ωthat is smaller than, but of order, unity. Thus these theories may solve the quasi-flatness problem, a considerably more challenging problem than the flatness problem. We also discuss the related Λand quasi-Λproblem in these theories. We conclude with an appraisal of the difficulties these theories may face.

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arXiv1999

A time varying speed of light as a solution to cosmological puzzles

We consider the cosmological implications of light travelling faster in the early Universe. We propose a prescription for deriving corrections to the cosmological evolution equations while the speed of light $c$ is changing. We then show how the horizon, flatness, and cosmological constant problems may be solved. We also study cosmological perturbations in this scenario and show how one may solve the homogeneity and isotropy problems. As it stands, our scenario appears to most easily produce extreme homogeneity, requiring structure to be produced in the Standard Big Bang epoch. Producing significant perturbations during the earlier epoch would require a rather careful design of the function $c(t)$. The large entropy inside the horizon nowadays can also be accounted for in this scenario.

astro-ph↗

arXiv1998

Varying-$α$ Theories and Solutions to the Cosmological Problems

If the fine structure constant $α=e^2/(\hbar c)$ were to change, then a number of interpretations would be possible, attributing this change either to variations in the electron charge, the dielectric constant of the vacuum, the speed of light, or Planck's constant. All these variations should be operationally equivalent and can be related by changes of standard units. We show how the varying speed of light cosmology recently proposed can be rephrased as a dielectric vacuum theory, similar to the one proposed by Bekenstein. The cosmological problems will therefore also be solved in such a theory.

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arXiv1998

CMB and LSS Power Spectra From Local Cosmic String Seeded Struture Formation

We evaluate the two point functions of the stress energy from the largest string simulations carried out so far. The two point functions are used to calculate the cosmic microwave background (CMB) and cold dark matter (CDM) power spectra from local cosmic string models for structure formation. We find that our spectra differ significantly from those previously calculated for both global and local defects. We find a higher Doppler peak at $l=400-600$ and a less severe bias problem than for global defects. Spectra were obtained for a variety of network energy-decay mechanisms.

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arXiv1998

CMB and density fluctuations from strings plus inflation

In cosmological models where local cosmic strings are formed at the end of a period of inflation, the perturbations are seeded both by the defects and by the quantum fluctuations. In a subset of these models, for example those based on $D$-term inflation, the amplitudes are similar. Using our recent calculations of structure formation with cosmic strings, we point out that in a flat cosmology with zero cosmological constant and 5% baryonic component, strings plus inflation fits the observational data much better than each component individually. The large-angle CMB spectrum is mildly tilted, for Harrison-Zeldovich inflationary fluctuations. It then rises to a thick Doppler bump, covering $\ell=200-600$, modulated by soft secondary undulations. The standard CDM anti-biasing problem is cured, giving place to a slightly biased scenario of galaxy formation.

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arXiv1998

The power spectra of CMB and density fluctuations seeded by local cosmic strings

We compute the power spectra in the cosmic microwave background and cold dark matter (CDM) fluctuations seeded by strings, using the largest string simulations performed so far to evaluate the two-point functions of their stress energy tensor. We find that local strings differ from global defects in that the scalar components of the stress-energy tensor dominate over vector and tensor components. This result has far reaching consequences. We find that cosmic strings exhibit a single Doppler peak of acceptable height at high $\ell$. They also seem to have a less severe bias problem than global defects, although the CDM power spectrum in the ``standard'' cosmology (flat geometry, zero cosmological constant, 5% baryonic component) is the wrong shape to fit large scale structure data.

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