arXiv subjects

Subir Sarkar

Publications and source records attributed to Subir Sarkar.

At least 109 records · Page 6Linked to original sources

arXiv2004

A "Baedecker" for the Dark Matter Annihilation Signal

We provide a ``Baedecker'' or travel guide to the directions on the sky where the dark matter annihilation signal may be expected. We calculate the flux of high energy gamma-rays from annihilation of neutralino dark matter in the centre of the Milky Way and the three nearest dwarf spheroidals (Sagittarius, Draco and Canis Major), using realistic models of the dark matter distribution. Other investigators have used cusped dark halo profiles (such as the Navarro-Frenk-White) to claim a significant signal. This ignores the substantial astrophysical evidence that the Milky Way is not dark-matter dominated in the inner regions. We show that the annihilation signal from the Galactic Centre falls by two orders of magnitude on substituting a cored dark matter density profile for a cusped one. The present and future generation of high energy gamma-ray detectors, whether atmospheric Cerenkov telescopes or space missions like GLAST, lack the sensitivity to detect any of the monochromatic gamma-ray annihilation lines. The continuum gamma-ray signal above 1 GeV and above 50 GeV may however be detectable either from the dwarf spheroidals or from the Milky Way itself. If the density profiles of the dwarf spheroidals are cusped, then the best prospects are for detecting Sagittarius and Canis Major. However, if the dwarf spheroidals have milder, cored profiles, then the annihilation signal is not detectable. For GLAST, an attractive strategy is to exploit the wide field of view and observe the Milky Way at medium latitudes, as suggested by Stoehr et al. This is reasonably robust against changes in the density profile.

astro-ph↗

arXiv2003

New physics from ultrahigh energy cosmic rays

Observations of cosmic rays with energies above ~ 4 x 10^{10} GeV have inspired several speculative suggestions concerning their origin. The crucial question is whether or not the spectrum exhibits the expected `GZK cutoff' at this energy -- concerning which there are presently contradictory results. If there is indeed a cutoff, then the sources are cosmologically distant and rather exotic in nature. If there is no cutoff then new physics is required.

hep-ph↗

arXiv2003

An alternative to the cosmological 'concordance model'

Precision measurements of the cosmic microwave background by WMAP are believed to have established a flat $Λ$-dominated universe, seeded by nearly scale-invariant adiabatic primordial fluctuations. However by relaxing the hypothesis that the fluctuation spectrum can be described by a single power law, we demonstrate that an Einstein-de Sitter universe with {\em zero} cosmological constant can fit the data as well as the best concordance model. Moreover unlike a $Λ$-dominated universe, such an universe has no strong integrated Sachs-Wolfe effect, so is in better agreement with the low quadrupole seen by WMAP. The main problem is that the Hubble constant is required to be rather low: $H_0 \simeq 46$ km/s/Mpc; we discuss whether this can be consistent with observations. Furthermore for universes consisting only of baryons and cold dark matter, the amplitude of matter fluctuations on cluster scales is too high, a problem which seems generic. However, an additional small contribution ($Ω_X \sim 0.1$) of matter which does not cluster on small scales, e.g. relic neutrinos with mass of order eV or a `quintessence' with $w \sim 0$, can alleviate this problem. Such models provide a satisfying description of the power spectrum derived from the 2dF galaxy redshift survey and from observations of the Ly-$α$ forest. We conclude that Einstein-de Sitter models can indeed accommodate all data on the large scale structure of the Universe, hence the Hubble diagram of distant Type Ia supernovae remains the only {\em direct} evidence for a non-zero cosmological constant.

astro-ph↗

arXiv2003

The clustering of ultra-high energy cosmic rays and their sources

The sky distribution of cosmic rays with energies above the 'GZK cutoff' holds important clues to their origin. The AGASA data, although consistent with isotropy, shows evidence for small-angle clustering, and it has been argued that such clusters are aligned with BL Lacertae objects, implicating these as sources. It has also been suggested that clusters can arise if the cosmic rays come from the decays of very massive relic particles in the Galactic halo, due to the expected clumping of cold dark matter. We examine these claims and show that both are in fact not justified.

astro-ph↗

arXiv2003

Neutrinos from the Big Bang

The standard Big Bang cosmology predicts the existence of an, as yet undetected, relic neutrino background, similar to the photons observed in the cosmic microwave background. If neutrinos have mass, then such relic neutrinos are a natural candidate for the dark matter of the universe, and indeed were the first particles to be proposed for this role. This possibility has however been increasingly constrained by cosmological considerations, particularly of large-scale structure formation, thus yielding stringent bounds on neutrino masses, which have yet to be matched by laboratory experiments. Another probe of relic neutrinos is primordial nucleosynthesis which is sensitive to the number of neutrino types (including possible sterile species) as well to any lepton asymmetry. Combining such arguments with the experimental finding that neutrino mixing angles are large, excludes the possibility of a large asymmetry and disfavours new neutrinos beyond those now known.

hep-ph↗

arXiv2002

Measuring the baryon content of the universe: BBN vs CMB

The relic abundance of baryons - the only form of stable matter whose existence we are certain of - is a crucial parameter for many cosmological processes, as well as material evidence that there is new physics beyond the Standard Model. We discuss recent determinations of the cosmological baryon density from analysis of the abundances of light elements synthesised at the end of ``the first three minutes'', and from the observed temperature anisotropies imprinted on small angular-scales in the cosmic microwave background when the universe was about 100,000 yr old.

astro-ph↗

arXiv2002

Possible astrophysical probes of quantum gravity

A satisfactory theory of quantum gravity will very likely require modification of our classical perception of space-time, perhaps by giving it a 'foamy' structure at scales of order the Planck length. This is expected to modify the propagation of photons and other relativistic particles such as neutrinos, such that they will experience a non-trivial refractive index even in vacuo. The implied spontaneous violation of Lorentz invariance may also result in alterations of kinematical thresholds for key astrophysical processes involving high energy cosmic radiation. We discuss experimental probes of these possible manifestations of the fundamental quantum nature of space-time using observations of distant astrophysical sources such as gamma-ray bursts and active galactic nuclei.

gr-qc↗

arXiv2002

New results in cosmology

From an observational perspective cosmology is today in excellent shape - advances in instrumentation and data processing have enabled us to study the universe in detail back to when the first galaxies formed, map the fluctuations in the cosmic microwave background which provide a measure of the overall geometry, and reconstruct the thermal history reliably back to at least the primordial nucleosynthesis era. However recent deep studies of the Hubble expansion rate have suggested that the universe is accelerating, driven by some form of `dark' (vacuum) energy. If true, this implies a new energy scale in Nature of order 0.001 eV, well below any known scale of fundamental physics. This has refocussed attention on the notorious cosmological constant problem at the interface of general relativity and quantum field theory. It is possible that the resolution of this situation will require fundamental modifications to our ideas about gravity.

hep-ph↗

arXiv2002

Ultra-high energy cosmic rays and new physics

Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin `cutoff' at $\sim 4 \times 10^{10}$ GeV pose a conundrum, the solution of which requires either drastic revision of our astrophysical understanding, or new physics beyond the Standard Model. Nucleons of such energies must originate within the local supercluster in order to avoid excessive energy losses through photopion production on the cosmic microwave background. However they do not point back towards possible nearby sources, e.g. the active galaxy Cen A or M87 in the Virgo cluster, so such an astrophysical origin requires intergalactic magnetic fields to be a hundred times stronger than previously believed, in order to isotropise their arrival directions. Alternatively the primaries may be high energy neutrinos, say from distant gamma-ray bursts, which annihilate on the local relic background neutrinos to create ``Z-bursts''. A related possibility is that the primary neutinos may initiate the observed air showers directly if their interaction cross-sections are boosted to hadronic strength through non-perturbative physics such as TeV-scale quantum gravity. Or the primaries may instead be new strongly interacting neutral particles with a longer mean free path than nucleons, coming perhaps from distant BL-Lac objects or FR-II radio galaxies. Yet another possibility is that Lorentz invariance is violated at high energies thus suppressing the energy loss processes altogether. The idea that has perhaps been studied in most detail is that such cosmic rays originate from the decays of massive relic particles (``wimpzillas'') clustered as dark matter in the galactic halo. All these hypotheses will soon be critically tested by the Pierre Auger Observatory, presently under construction in Argentina, and by proposed satellite experiments such as EUSO.

hep-ph↗

arXiv2001

The high energy cosmic ray spectrum from relic particle decay

It has been speculated that the recently detected ultra-high energy cosmic rays may originate from the decays of relic particles with mass of order $10^{12}$ GeV clustered in the halo of our Galaxy. This hypothesis can be tested through forthcoming measurements of the spectra of both high energy cosmic nucleons and neutrinos, which are determined in this model by the physics of QCD fragmentation, with no astrophysical uncertainties. We evolve fragmentation spectra measured at LEP energies up to the scale of the decaying particle mass by numerical solution of the DGLAP equations. This enables incorporation of the effects of supersymmetry on the development of the cascade and we also allow for decays into many-particle states. The calculated spectral shape agrees well with present cosmic ray data beyond the Greisen-Zatsepin-Kuzmin energy.

hep-ph↗

arXiv2001

No cosmological domain wall problem for weakly coupled fields

After inflation occurs, a weakly coupled scalar field will in general not be in thermal equilibrium but have a distribution of values determined by the inflationary Hubble parameter. If such a field subsequently undergoes discrete symmetry breaking, then the different degenerate vacua may not be equally populated so the domain walls which form will be `biased' and the wall network will subsequently collapse. Thus the cosmological domain wall problem may be solved for sufficiently weakly coupled fields in a post-inflationary universe. We quantify the criteria for determining whether this does happen, using a Higgs-like potential with a spontaneously broken $Z_2$ symmetry.

hep-ph↗

arXiv2001

The Anisotropy of the Ultra-High Energy Cosmic Rays

Ultra-high energy cosmic rays (UHECRs) may originate from the decay of massive relic particles in the dark halo of the Galaxy, or they may be produced by supermassive black holes in the nuclei of nearby galaxies. The anisotropy in the arrival directions is studied in four dark halo models (cusped, isothermal, triaxial and tilted) and in four galaxy samples (galaxies intrinsically brighter than Centaurus A within 50 and 100 Mpc, and brighter than M32 within 50 and 100 Mpc). In decaying dark matter models, the amplitude of the anisotropy is controlled by the size of the Galactic halo, while the phase is controlled by the shape. In the northern hemisphere, the amplitude is about 0.5 for cusped haloes, but falls to roughly 0.3 for isothermal haloes. The phase points in the direction of the Galactic Centre, with deviations of up to 30 degrees possible for triaxial and tilted haloes. The effect of the halo of M31 is too weak to provide conclusive evidence for the decaying dark matter origin of UHECRs. In extragalactic models, samples of galaxies brighter than Centaurus A produce substantial anisotropies (roughly 1.8), much larger than the limits set by the available data. If all galaxies brighter than M32 contribute, then the anisotropy is more modest (roughly 0.5) and is directed toward mass concentrations in the supergalactic plane, like the Virgo cluster. Predictions are made for the southern hemisphere station of the Pierre Auger Observatory. If the UHECRs have a Galactic origin, then the phase points towards the Galactic Centre. If they have an extragalactic origin, then it points in the rough direction of the Fornax cluster. This provides an unambiguous discriminant and requires about 350-500 events at South Auger.

astro-ph↗

arXiv2001

Low-scale inflation

We show that the scale of the inflationary potential may be the electroweak scale or even lower, while still generating an acceptable spectrum of primordial density perturbations. Thermal effects readily lead to the initial conditions necessary for low scale inflation to occur, and even the moduli problem can be evaded if there is such an inflationary period. We discuss how low scale inflationary models may arise in supersymmetric theories or in theories with large new space dimensions.

hep-ph↗

arXiv2001

On the APM power spectrum and the CMB anisotropy: Evidence for a phase transition during inflation?

Adams et al. (1997b) have noted that according to our current understanding of the unification of fundamental interactions, there should have been phase transitions associated with spontaneous symmetry breaking {\em during} the inflationary era. This may have resulted in the breaking of scale-invariance of the primordial density perturbation for brief periods. A possible such feature was identified in the power spectrum of galaxy clustering in the APM survey at the scale $k \sim 0.1 h$ Mpc^{-1} and it was shown that the secondary acoustic peaks in the power spectrum of the CMB anisotropy should consequently be suppressed. We demonstrate that this prediction is confirmed by the recent Boomerang and Maxima observations, which favour a step-like spectral feature in the range $k \sim (0.06-0.6)h$ Mpc^{-1}, independently of the similar previous indication from the APM data. Such a spectral break enables an excellent fit to both APM and CMB data with a baryon density consistent with the BBN value. It also allows the possibility of a matter-dominated universe with zero cosmological constant, which we show can now account for even the evolution of the abundance of rich clusters.

astro-ph↗

arXiv2000

Thermalisation after inflation

During (re)heating of the universe after inflation, the relativistic decay products of the inflaton field $ϕ$ must lose energy and additional particles must be produced to attain a thermalised state at a temperature $T_{\reh}$. We estimate the rate of energy loss via elastic and inelastic scattering interactions. Elastic scattering is an inefficient energy loss mechanism so inelastic processes, although higher order in the coupling $α$, can be faster because more energy is transfered. The timescale to produce a particle number density of ${\cal O}(T_{\reh}^3)$ is the inelastic energy loss timescale, $\sim(α^3 n_ϕ/T_{\reh}^2)^{-1}$.

hep-ph↗

arXiv2000

Cosmic ray signatures of massive relic particles

The possibility that the Fermi scale is the only fundamental energy scale of Nature is under serious consideration at present, yet cosmic rays may already have provided direct evidence of new physics at a much higher scale. The recent detection of very high energy particles with no plausible astrophysical sources suggests that these originate from the slow decays of massive particles clustered in the halo of our Galaxy. Such particles had in fact been predicted to exist beforehand with mass and lifetime in the range required to explain the observations. I discuss recent work focussing on experimental tests of this speculative but exciting idea.

hep-ph↗

arXiv2000

A supersymmetric solution to the KARMEN time anomaly

We interpret the KARMEN time anomaly as being due to the production of a (dominantly bino) neutralino with mass 33.9 MeV, which is the lightest supersymmetric particle but decays into 3 leptons through the violation of R-parity. For independent gaugino masses M_1 and M_2 we find regions in the (M_1, M_2, mu, tan beta) parameter space where such a light neutralino is consistent with all experiments. Future tests of this hypothesis are outlined.

hep-ph↗

arXiv1999

Implementing quadratic supergravity inflation

We study inflation driven by a slow-rolling inflaton field, characterised by a quadratic potential, and incorporating radiative corrections within the context of supergravity. In this model the energy scale of inflation is not overly constrained by the requirement of generating the observed level of density fluctuations and can have a physically interesting value, e.g. the supersymmetry breaking scale of $10^{10}$ GeV or the electroweak scale of $10^3$ GeV. In this mass range the inflaton is light enough to be confined at the origin by thermal effects, naturally generating the initial conditions for a (last) stage of inflation of the new inflationary type.

hep-ph↗