arXiv subjects

Richard Lieu

Publications and source records attributed to Richard Lieu.

At least 37 records · Page 2Linked to original sources

arXiv2016

Are fast radio bursts the birthmark of magnetars?

A model of fast radio bursts, which enlists young, short period extragalactic magnetars satisfying $B/P > 2 \times 10^{16}$~G~s$^{-1}$ (1~G = 1~statvolt~cm$^{-1}$) as the source, is proposed. When the parallel component $\bE_\parallel$ of the surface electric field (under the scenario of a vacuum magnetosphere) of such pulsars approaches 5 \% of the critical field $E_c = m_e^2 c^3/(e\hbar)$, in strength, the field can readily decay via the Schwinger mechanism into electron-positron pairs, the back reaction of which causes $\bE_\parallel$ to oscillate on a characteristic timescale smaller than the development of a spark gap. Thus, under this scenario, the open field line region of the pulsar magnetosphere is controlled by Schwinger pairs, and their large creation and acceleration rates enable the escaping pairs to coherently emit radio waves directly from the polar cap. The majority of the energy is emitted at frequencies $\lesssim 1$~GHz where the coherent radiation has the highest yield, at a rate large enough to cause the magnetar to lose spin significantly over timescale $\ap$ a few $\times 10^{-3}$~s, the duration of a fast radio burst. Owing to circumstellar environment of a young magnetar, however, the $\lesssim 1$~GHz radiation is likely to be absorbed or reflected by the overlying matter. It is shown that the brightness of the remaining (observable) frequencies of $\ap 1$~GHz and above are on par with a typical fast radio burst. Unless some spin-up mechanism is available to recover the original high rotation rate that triggered the Schwinger mechanism, the fast radio burst will not be repeated again in the same magnetar.

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arXiv2015

Measuring the brightness of classical noise dominated light at the shot noise limit?

A recent claim by Lieu et al that beam splitter intensity subtraction (or homodyne with one vacuum port) followed by high resolution sampling can lead to detection of brightness of thermal light at the shot noise limit is reexamined here. We confirm the calculation of Zmuidzinas that the claim of Lieu et al was falsified by an incorrect assumption about the correlations in thermal noise.

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arXiv2014

A method to improve the sensitivity of radio telescopes

As an extension of the ideas of Hanbury-Brown and Twiss, a method is proposed to eliminate the phase noise of white chaotic light in the regime where it is dominant, and to measure the much smaller Poisson fluctuations from which the incoming flux can be reconstructed (via the equality between variance and mean). The best effect is achieved when the timing resolution is finer than the inverse bandwidth of the spectral filter. There may be applications to radio astronomy at the phase noise dominated frequencies of $1 - 10$GHz, in terms of potentially increasing the sensitivity of telescopes by an order of magnitude.

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arXiv2013

Measurement of the dispersion of radiation from a steady cosmological source

The `missing baryons' of the near universe are believed to be principally in a partially ionized state. Although passing electromagnetic waves are dispersed by the plasma, the effect has hitherto not been utilized as a means of detection because it is generally believed that a successful observation requires the background source to be highly variable, \ie~the class of sources that could potentially deliver a verdict is limited. We argue in two stages that this condition is not necessary. First, by modeling the fluctuations on macroscopic scales as interference between wave packets we show that, in accordance with the ideas advanced by Einstein in 1917, both the behavior of photons as bosons (\ie~the intensity variance has contributions from Poisson and phase noise) and the van-Cittert-Zernike theorem are a consequence of wave-particle duality. Nevertheless, we then point out that in general the variance on some macroscopic timescale $τ$ consists of (a) a main contributing term $\propto 1/τ$, plus (b) a small negative term $\propto 1/τ^2$ due to the finite size of the wave packets. If the radiation passes through a dispersive medium, this size will be enlarged well beyond its vacuum minimum value of $Δt \approx 1/Δν$, leading to a more negative (b) term (while (a) remains unchanged) and hence a suppression of the variance w.r.t. the vacuum scenario. The phenomenon, which is typically at the few parts in 10$^5$ level, enables one to measure cosmological dispersion in principle. Signal-to-noise estimates, along with systematic issues and how to overcome them, will be presented.

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arXiv2013

Has inflation really solved the problems of flatness and absence of relics?

Among the three cosmological enigma solved by the theory of inflation, {\it viz.} (a) large scale flatness, (b) absence of monopoles and strings, and (c) structure formation, the first two are addressed from the viewpoint of the observed scales having originated from very small ones, on which the density fluctuations of the curvaton and relics are {\it inevitably} of order unity or larger. By analyzing strictly classically (and in two different gauges to ensure consistency) the density evolution of the smoothest possible pre-inflationary component -- thermal radiation -- it is found that the O(1) statistical fluctuations on the thermal wavelength scale present formidable obstacles to the linear theory of amplitude growth by the end of inflation. Since this wavelength scale exited the horizon at an early stage of inflation, it severely limits the number of e-folds of perturbative inflation. With more e-folds than $\approx 60$ there will be even larger fluctuations in the radiation density that ensures inflation keeps making `false starts'. The only `way out' is to invoke a super-homogeneous pre-inflationary fluid, at least on small scales, adding to the fine-tuning and preventing one from claiming that inflation simply `redshifts away' all the relic inhomogeneities; {\it i.e} the theory actually provided no explanation of (a) or (b), merely a tautology.

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arXiv2013

Large pre-inflationary thermal density perturbations

In some versions of the theory of inflation, it is assumed that before inflation began the universe was in a Friedmann-Robertson-Walker (FRW) stage, with the energy density dominated by massless particles. The origin of the nearly scale-invariant density perturbations is quantum fluctuations in the inflaton field. Here we point out that under those conditions there would necessarily also be large thermally induced density perturbations. It is asserted that inflation would smooth out any pre-existing perturbations. But that argument relies on linear perturbation theory of the scalar modes, which would be rendered invalid because of the non-negligibility of the vector and tensor modes when the perturbation in the total density becomes large. Under those circumstances the original proof that inflation would have the desired smoothing effect no longer applies, {\it i.e.} for the theory to be robust an alternative (and hitherto unavailable) demonstration of the smoothing that takes account of these non-linear terms is necessary.

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arXiv2013

A new way of detecting intergalactic baryons

For each photon wave packet of extragalactic light, the dispersion by line-of-sight intergalactic plasma causes an increase in the envelope width and a chirp (drift) in the carrier frequency. It is shown that for continuous emission of many temporally overlapping wave packets with random epoch phases, such as quasars in the radio band, this in turn leads to quasi-periodic variations in the intensity of the arriving light on timescales between the coherence time (defined as the reciprocal of the bandwidth of frequency selection, taken here as of order 0.01 GHz for radio observations) and the stretched envelope, with most of the fluctuation power on the latter scale which is typically in the millisecond range for intergalactic dispersion. Thus, by monitoring quasar light curves on such short scales, it should be possible to determine the line-of-sight plasma column along the many directions and distances to the various quasars, affording one a 3-dimensional picture of the ionized baryons in the near universe.

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arXiv2009

Significant foreground unrelated non-acoustic anisotropy on the one degree scale in WMAP 5-year observations

The spectral variation of the cosmic microwave background (CMB) as observed by WMAP was tested using foreground reduced WMAP5 data, by producing subtraction maps at the 1$^\circ$ angular resolution between the two cosmological bands of V and W, for masked sky areas that avoid the Galactic disk. The resulting $V-W$ map revealed a non-acoustic signal over and above the WMAP5 pixel noise, with two main properties. Firstly, it possesses quadrupole power at the $\approx$ 1 $μK$ level which may be attributed to foreground residuals. Second, it fluctuates also at all values of $\ell >$ 2, especially on the $1^\circ$ scale ($200 \lesssim \ell \lesssim 300$). The behavior is {\it random and symmetrical} about zero temperature with a r.m.s. amplitude of $\approx$ 7 $μK$, or 10 % of the maximum CMB anisotropy, which would require a `cosmic conspiracy' among the foreground components if it is a consequence of their existences. Both anomalies must be properly diagnosed and corrected if `precision cosmology' is the claim. The second anomaly is, however, more interesting because it opens the question on whether the CMB anisotropy genuinely represents primordial density seeds.

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arXiv2009

The nature of intrinsic fluctuations in cosmic diffuse radiation

The spatial and temporal noise properties of diffuse radiation is investigated in the context of the cosmic microwave background (CMB), although generic formulae that enable application to any other forms of incoherent light of a prescribed energy spectrum are also provided. It is shown that the variance of fluctuations in the density and flux consists of two parts. First is a term from the spontaneous emission coefficient which is the contribution from a random gas of classical particles representing the corpuscular (photon) nature of light. Second is a term from the stimulated emission coefficient which leads to a `wave noise', or more precisely the noise arising from the superposition of many plane waves of arbitrary phase - the normal modes of the radiation. The origin of this second term has never been elucidated before. We discussed one application. In the spatially homogeneous post-inflationary epochs when the universe was reheated to GUT temperatures, the thermal CMB density fluctuations on the horizon scale is of order the WMAP measured value of 10$^{-5}$. Beyond (larger than) the horizon, the power spectrum of perturbations could moreover assume the observed form of $P(k) \sim k$ if thermal diffusion to equilibrium does not merely involve classical particles, but also non-localized wave components. The outcome of our study clearly demonstrates that such components do indeed exist.

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arXiv2009

A natural origin of primordial density perturbations

We suggest here a mechanism for the seeding of the primordial density fluctuations. We point out that a process like reheating at the end of inflation will inevitably generate perturbations, even on superhorizon scales, by the local diffusion of energy. Provided that the reheating temperature is of order the GUT scale, the density contrast $δ_R$ for spheres of radius $R$ will be of order $10^{-5}$ at horizon entry, consistent with the values measured by \texttt{WMAP}. If this were a purely classical process, $δ_R^2$ would fall as $1/R^4$ beyond the horizon, and the resulting primordial density power spectrum would be $P(k) \propto k^n$ with $n=4$. However, as shown by Gabrielli et al, a quantum diffusion process can generate a power spectrum with any index in the range $0 1$). Thus, the two characteristic parameters that determine the appearance of present day structures could be natural consequences of this mechanism. These are in any case the minimum density variations that must have formed if the universe was rapidly heated to GUT temperatures by the decay of a `false vacuum'. There is then no \emph{a priori} necessity to postulate additional (and fine tuned) quantum fluctuations in the `false vacuum', nor a pre-inflationary period. Given also the very stringent pre-conditions required to trigger a satisfactory period of inflation, altogether it seems at least as natural to assume that the universe began in a flat and homogeneously expanding phase.

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arXiv2009

The diffuse soft excess emission in the Coma cluster from the ROSAT All-Sky Survey

RASS data near the North Galactic Pole was analyzed in order to study the large-scale distribution of soft X-ray emission from the Coma cluster. These RASS data constitute the only available X-ray observations of Coma that feature an in situ -- temporally and spatially contiguous -- background, with unlimited and continuous radial coverage. These unique characteristics of the RASS data are used to deliver a final assessment on whether the soft excess previously detected in the Coma cluster is due to background subtraction errors, or not. This paper confirms the presence of soft X-ray excess associated with Coma, and reports the detection of 1/4 keV band excess out to 5 Mpc from the cluster center, the largest soft excess halo discovered to date. We propose that the emission is related to filaments that converge towards Coma, and generated either by non-thermal radiation caused by accretion shocks, or by thermal emission from the filaments themselves.

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arXiv2009

Soft X-ray excess of clusters: a thermal filament model, and the strong lensing of background galaxy groups

The observational and theoretical status of the search for missing cosmological baryons is summarized, with a discussion of some indirect methods of detection. The thermal interpretation of the cluster soft X-ray and EUV excess phenomenon is examined in the context of emission filaments, which are the higher density part of the warm hot intergalactic medium (WHIM) residing at the outskirt of clusters. We derived an analytic radial profile of the soft excess surface brightness using a simple filament model, which provided us a means of observationally constraining the WHIM parameters, especially the total mass budget of warm gas associated with a cluster. We then pointed out a new scenario for soft excess emission, viz. a cluster that can strongly lens the soft X-rays from background WHIM knots. If, as seems quite likely, the missing baryons are mostly in the WHIM halos of galaxy groups, the lensing probability will be quite high ($\sim$ 10 %). This way of accounting for at least part of a cluster's soft excess may also explain the absence of O VII absorption at the redshift of the cluster.

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arXiv2008

Hubble redshift and the Heisenberg frequency uncertainty: on a coherence (or pulse) time signature in extragalactic light

In any Big Bang cosmology, the frequency $ω$ of light detected from a distant source is continuously and linearly changing (usually redshifting) with elapsed observer's time $δt$, because of the expanding Universe. For small $δt$, however, the resulting $δω$ shift lies beneath the Heisenberg frequency uncertainty. And since there {\it is} a way of telling whether such short term shifts really exist, if the answer is affirmative we will have a means of monitoring radiation to an accuracy level that surpasses fundamental limitations. More elaborately, had $ω$ been `frozen' for a minimum threshold interval before any redshift could take place, i.e. the light propagated as a smooth but {\it periodic} sequence of wave packets or pulses, and $ω$ decreased only from one pulse to the next, one would then be denied the above forbiddingly precise information about frequency behavior. Yet because this threshold period is {\it observable}, being $Δt \approx 1/\sqrt{ω_0 H_0} \sim$ 5 -- 15 minute for the cosmic microwave background (CMB), we can indeed perform a check for consistency between the Hubble Law and the Uncertainty Principle. If, as most would assume to be the case, the former either takes effect without violating the latter or not take effect at all, the presence of this characteristic time signature (periodicity) $Δt$ would represent direct verification of the redshift phenomenon.

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arXiv2007

On the absence of shear from perfect Einstein rings and the stability of geometry

Concordance cosmology points to a Universe of zero mean curvature, due to the inflation mechanism which occurred soon after the Big Bang, while along a relatively small number of lower redshift light paths where lensing events are observed, space is positively curved. How do we know that global geometry and topology are robust rather than in a state of chaos? The phenomenon of cosmic shear provides an effective way of mapping curvature fluctuations, because it affects {\it any} light rays whether they intercept mass clumps or not. We discuss a range of astrophysical applications of the principal manifestation of shear - the distortion of images. It will be shown that the quickest way of testing the existence of shear in the near Universe is to look at the shape of Einstein rings. The fact that most of these rings are circular to a large extent means, statistically speaking, shear occurs at a much lower level than the expectation based upon our current understanding of the inhomogeneous Universe. While inflation may account for the mean geometry, it offers no means of stabilizing it against the fluctuations caused by non-linear matter clumping at low redshift. Either this clumping is actually much less severe, or the physical mechanism responsible for shaping the large scale curvature has been active not only during the very early epochs, but also at all subsequent times. Might it be the vital `interface' between expansion on Hubble distances and gravity on cluster scales and beneath?

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arXiv2007

Strong lensing time delay: a new way of measuring cosmic shear

The phenomenon of cosmic shear, or distortion of images of distant sources unaccompanied by magnification, is an effective way of probing the content and state of the foreground Universe, because light rays do not have to pass through mass structures in order to be sheared. It is shown that the delay in the arrival times between two simultaneously emitted photons that appear to be arriving from a pair of images of a strongly lensed cosmological source contains not only information about the Hubble constant, but also the long range gravitational effect of galactic scale mass clumps located away from the light paths in question. This is therefore also a method of detecting shear. Data on time delays among a sample of strongly lensed sources can provide crucial information about whether extra dynamics beyond gravity and dark energy are responsible for the global flatness of space. If the standard $ΛCDM$ model is correct, there should be a large dispersion in the value of $H_0$ as inferred from the delay data by (the usual procedure of) ignoring the effect of all other mass clumps except the strong lens itself. The fact that there has not been any report of a significant deviation from the $h =$ 0.7 mark during any of the $H_0$ determinations by this technique may already be pointing to the absence of the random effect discussed here.

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arXiv2007

Time delay by primordial density fluctuations: its biasing effect on the observed mean curvature of the Universe

In this paper we specifically studied one aspect of foreground primordial matter density perturbations: the relative gravitational time delay between a pair of light paths converging towards an observer and originating from two points on the last scattering surface separated by the physical scale of an acoustic oscillation. It is found that time delay biases the size of acoustic oscillations {\it systematically} towards smaller angles, or larger harmonic numbers $\ell$, i.e. the mean geometry as revealed by CMB light becomes that of an open Universe if $Ω=1$. Since the effect is second order, its standard deviation $δ\ell/\ell \sim (δΦ)^2$ where $(δΦ)^2 \sim 10^{-9}$ is the normalization of the primordial matter spectrum $P(k)$, the consequence is too numerically feeble to warrant a re-interpretion of WMAP data. If, however, this normalization were increased to $δΦ\gtrsim 0.01$ which is still well within the perturbation limit, the shift in the positions of the acoustic peaks would have been substantial enough to implicate inflationary $Λ$CDM cosmology. Thus $Ω$ is not the only parameter (and by deduction inflation cannot be the only mechanism) of relevance to the understanding of {\it observed} large scale geometry. The physics that explains why $δΦ$ is so small also plays a crucial role, but since this is a separate issue independent of inflation, might it be less artificial to look for an alternative solution to the flatness problem altogether?

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arXiv2007

$Λ$CDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?

Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter {\it and now} dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of $Λ$CDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind $Λ$CDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark.

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arXiv2006

The outermost gravitationally bound orbit around a mass clump in an expanding Universe: implication on rotation curves and dark matter halo sizes

Conventional treatment of cold dark matter halos employs the Navarro-Frenk-White (NFW) profile with a maximum radius set at $r=r_{200}$, where the enclosed matter has an overdensity of 200 times the critical density. The choice of $r=r_{200}$ is somewhat arbitrary. It is not the collapsed (virial) radius, but does give $r \sim$ 1 Mpc for rich clusters, which is a typical X-ray size. Weak lensing measurements, however, reveal halo radii well in excess of $r_{200}$. Is there a surface that places an absolute limit on the extension of a halo? To answer the question, we derived analytically the solution for circular orbits around a mass concentration in an expanding flat Universe, to show that an outermost orbit exists at $v/r = H$, where $v$ is the orbital speed and $H$ is the Hubble constant. The solution, parametrized as $r_2$, is independent of model assumptions on structure formation, and {\it is the radius at which the furthest particle can be regarded as part of the bound system}. We present observational evidence in support of dark matter halos reaching at least as far out as $r=r_2$. An interesting consequence that emerges concerns the behavior of rotation curves. Near $r=r_2$ velocities will be biased low. As a result, the mass of many galaxy groups may have been underestimated. At $r=r_2$ there is an abrupt cutoff in the curve, irrespective of the halo profile. An important cosmological test can therefore be performed if velocity disperion data are available out to 10 Mpc radii for nearby clusters (less at higher redshifts). For Virgo it appears that there is no such cutoff.

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