Intergalactic Dust Research Articles

Coevolution of Dust and Chemistry in Galaxy Simulations with a Resolved Interstellar Medium

Nearby dwarf irregular galaxies are ideal laboratories for studying the interstellar medium (ISM) at low metallicity, which is expected to be common for galaxies at very high redshift being observed by JWST. We present the first high-resolution (∼0.2 pc) hydrodynamical simulations of an isolated low-metallicity (0.1 Z ⊙) dwarf galaxy coupled with a time-dependent chemistry network and a dust evolution model where dust is locally produced and destroyed by various processes. To accurately model carbon monoxide (CO), we post-process the simulations with a detailed chemistry network including the time-dependent effect of molecular hydrogen (H2). Our model successfully reproduces the observed star formation rate and CO(1–0) luminosity (L CO). We find that dust growth in dense gas is required to reproduce the observed L CO otherwise CO would be completely photodissociated. In contrast, the H2 abundance is extremely small and is insensitive to dust growth, leading to a CO-to-H2 conversion factor that is only slightly higher than the Milky Way value despite the low metallicity. An observationally inferred dust-to-gas ratio is thus underestimated if adopting the metallicity-dependent CO-to-H2 conversion factor. The newly produced dust in dense gas mixes with the ISM through supernova feedback without being completely destroyed by sputtering, which leads to galactic outflows 20%–50% dustier than the ISM, providing a possible source for intergalactic dust.

The Origin of Life on Earth, the Panspermia Hypothesis and Cosmological DNA Synthesis

The process of the rapid origin of life on Earth is analyzed, taking into account the evolution of our Sun (the beginning of thermonuclear reactions). Given the knowledge about DNA, viruses and the structure of bacteria, it is concluded that the origin of life on Earth was initiated from the outside. Further, using space chemistry, it is shown that spontaneous assembly of DNA/RNA molecules can occur on particles of interstellar and intergalactic dust. Therefore, when favorable conditions are created on the planet, the emergence of life begins rapidly, since the initiators of life are already available from cosmic dust (RNA and DNA molecules).

Universe opacity and Type Ia supernova dimming

ABSTRACT In this paper, I revoke a debate about an origin of Type Ia supernova (SN Ia) dimming. I argue that except for a commonly accepted accelerating expansion of the Universe, a conceivable alternative for explaining this observation is universe opacity caused by light extinction by intergalactic dust, even though it is commonly assumed that this effect is negligible. Using data of the Union2.1 SN Ia compilation, I find that the standard Λ cold dark matter (ΛCDM) model and the opaque universe model fit the SN Ia measurements at redshifts z < 1.4 comparably well. The optimum solution for the opaque universe model is characterized by the B-band intergalactic opacity $\lambda _{B} = 0.10 \pm 0.03 \, \mathrm{Gpc}^{-1}$ and the Hubble constant $H_0 = 68.0 \pm 2.5 \, \mathrm{km\, s^{-1}\, Mpc^{-1}}$. The intergalactic opacity is higher than that obtained from independent observations but still within acceptable limits. This result emphasizes that the issue of the accelerating expansion of the Universe as the origin of the SN Ia dimming is not yet definitely resolved. Obviously, the opaque universe model as an alternative to the ΛCDM model is attractive, because it avoids puzzles and controversies associated with dark energy and the accelerating expansion.

Universe opacity and CMB

A cosmological model, in which the cosmic microwave background (CMB) is a thermal radiation of intergalactic dust instead of a relic radiation of the Big Bang, is revived and revisited. The model suggests that a virtually transparent local Universe becomes considerably opaque at redshifts $z > 2-3$. Such opacity is hardly to be detected in the Type Ia supernova data, but confirmed using quasar data. The opacity steeply increases with redshift because of a high proper density of intergalactic dust in the previous epochs. The temperature of intergalactic dust increases as $(1+z)$ and exactly compensates the change of wavelengths due to redshift, so that the dust radiation looks apparently like the radiation of the blackbody with a single temperature. The predicted dust temperature is $T^{D} = 2.776 \, \mathrm{K}$, which differs from the CMB temperature by 1.9\% only, and the predicted ratio between the total CMB and EBL intensities is 13.4 which is close to 12.5 obtained from observations. The CMB temperature fluctuations are caused by EBL fluctuations produced by galaxy clusters and voids in the Universe. The polarization anomalies of the CMB correlated with temperature anisotropies are caused by the polarized thermal emission of needle-shaped conducting dust grains aligned by large-scale magnetic fields around clusters and voids. A strong decline of the luminosity density for $z > 4$ is interpreted as the result of high opacity of the Universe rather than of a decline of the global stellar mass density at high redshifts.

The cosmic transparency measured with Type Ia supernovae: implications for intergalactic dust

Abstract Observations of high-redshift Type Ia supernovae (SNe Ia) are used to study the cosmic transparency at optical wavelengths. Assuming a flat Λ cold dark matter (ΛCDM) cosmological model based on baryon acoustic oscillations and cosmic microwave background measurements, redshift dependent deviations of SN Ia distances are used to constrain mechanisms that would dim light. The analysis is based on the most recent Pantheon SN compilation, for which there is a $0.03 \pm 0.01 \, {(\rm {stat})}$ mag discrepancy in the distant supernova distance moduli relative to the ΛCDM model anchored by supernovae at z < 0.05. While there are known systematic uncertainties that combined could explain the observed offset, here we entertain the possibility that the discrepancy may instead be explained by scattering of supernova light in the intergalactic medium (IGM). We focus on two effects: Compton scattering by free electrons and extinction by dust in the IGM. We find that if the discrepancy is entirely due to dimming by dust, the measurements can be modelled with a cosmic dust density $\Omega _{\rm IGM}^{\rm dust} = 8 \times 10^{-5} (1+z)^{-1}$, corresponding to an average attenuation of 2 × 10−5 mag Mpc−1 in V band. Forthcoming SN Ia studies may provide a definitive measurement of the IGM dust properties, while still providing an unbiased estimate of cosmological parameters by introducing additional parameters in the global fits to the observations.

Missing dust signature in the cosmic microwave background

Abstract I examine a possible spectral distortion of the cosmic microwave background (CMB) due to its absorption by galactic and intergalactic dust. I show that even subtle intergalactic opacity of 1 × 10−7 mag h Gpc−1 at the CMB wavelengths in the local Universe causes non-negligible CMB absorption and decline of the CMB intensity because the opacity steeply increases with redshift. The CMB should be distorted even during the epoch of the Universe defined by redshifts z < 10. For this epoch, the maximum spectral distortion of the CMB is at least 20 × 10−22 W m−2 Hz−1 sr−1 at 300 GHz, which is well above the sensitivity of the COBE/FIRAS, WMAP or Planck flux measurements. If dust mass is considered to be redshift dependent with noticeable dust abundance at redshifts 2–4, the predicted CMB distortion would be even higher. The CMB would also be distorted in a perfectly transparent universe due to dust in galaxies, but this effect is lower by one order than that due to intergalactic opacity. The fact that the distortion of the CMB by dust is not observed is intriguing and questions either opacity and extinction law measurements or validity of the current model of the Universe.

Planckintermediate results

Although infrared (IR) overall dust emission from clusters of galaxies has been statistically detected using data from the Infrared Astronomical Satellite (IRAS), it has not been possible to sample the spectral energy distribution (SED) of this emission over its peak, and thus to break the degeneracy between dust temperature and mass. By complementing the IRAS spectral coverage with Planck satellite data from 100 to 857 GHz, we provide new constraints on the IR spectrum of thermal dust emission in clusters of galaxies. We achieve this by using a stacking approach for a sample of several hundred objects from the Planck cluster sample; this procedure averages out fluctuations from the IR sky, allowing us to reach a significant detection of the faint cluster contribution. We also use the large frequency range probed by Planck, together with component-separation techniques, to remove the contamination from both cosmic microwave background anisotropies and the thermal Sunyaev-Zeldovich effect (tSZ) signal, which dominate below 353 GHz. By excluding dominant spurious signals or systematic effects, averaged detections are reported at frequencies between 353 and 5000 GHz. We confirm the presence of dust in clusters of galaxies at low and intermediate redshifts, yielding an SED with a shape similar to that of the Milky Way. Planck's beam does not allow us to investigate the detailed spatial distribution of this emission (e.g., whether it comes from intergalactic dust or simply the dust content of the cluster galaxies), but the radial distribution of the emission appears to follow that of the stacked SZ signal, and thus the extent of the clusters. The recovered SED allows us to constrain the dust mass responsible for the signal, as well as its temperature. We additionally explore the evolution of the IR emission as a function of cluster mass and redshift.

Universe opacity and EBL

The observed extragalactic background light (EBL) is affected by light attenuation due to absorption of light by galactic and intergalactic dust in the Universe. Even galactic opacity of 10-20 percent and minute universe intergalactic opacity of $0.01\,\mathrm{mag}\,h\,\mathrm{Gpc}^{-1}$ at the local Universe have a significant impact on the EBL because obscuration of galaxies and density of intergalactic dust increase with redshift as $\left(1+z\right)^3$. Consequently, intergalactic opacity increases and the Universe becomes considerably opaque at $z > 3$. Adopting realistic values for galactic and intergalactic opacity, the estimates of the EBL for the expanding dusty universe are close to observations. The luminosity density evolution fits well measurements. The model reproduces a steep increase of the luminosity density at $z<2$, its maximum at $z=2-3$, and its decrease at higher redshifts. The increase of the luminosity density at low $z$ is not produced by the evolution of the star formation rate but by the fact that the Universe occupied a smaller volume in previous epochs. The decline of the luminosity density at high $z$ originates in the opacity of the Universe. The calculated bolometric EBL ranges from 100 to 200 $\mathrm{n W m}^{-2}\mathrm{sr}^{-1}$ and is within the limits of 40 and 200 $\mathrm{n W m}^{-2}\mathrm{sr}^{-1}$ of current EBL observations. The model predicts 98\% of the EBL coming from radiation of galaxies at $z<3.5$. Accounting for light extinction by intergalactic dust implies that the Universe was probably more opaque than dark for $z>3.5$.

THE DISTORTION OF THE COSMIC MICROWAVE BACKGROUND SPECTRUM DUE TO INTERGALACTIC DUST

ABSTRACT Infrared emission from intergalactic dust might compromise the ability of future experiments to detect subtle spectral distortions in the Cosmic Microwave Background (CMB) from the early universe. We provide the first estimate of foreground contamination of the CMB signal due to diffuse dust emission in the intergalactic medium. We use models of the extragalactic background light to calculate the intensity of intergalactic dust emission and find that emission by intergalactic dust at z ≲ 0.5 exceeds the sensitivity of the planned Primordial Inflation Explorer to CMB spectral distortions by 1–3 orders of magnitude. In the frequency range ν = 150–2400 GHz, we place an upper limit of 0.06% on the contribution to the far-infrared background from intergalactic dust emission.

Far-reaching dust distribution in galaxy discs

In most studies of dust in galaxies, dust is only detected from its emission to approximately the optical radius of the galaxy. By combining the signal of 110 spiral galaxies observed as part of the Herschel Reference Survey, we are able to improve our sensitivity by an order-of-magnitude over that for a single object. Here we report the direct detection of dust from its emission that extends out to at least twice the optical radius. We find that the distribution of dust is consistent with an exponential at all radii with a gradient of ~-1.7 dex R$_{25}^{-1}$. Our dust temperature declines linearly from ~25 K in the centre to 15 K at R$_{25}$ from where it remains constant out to ~2.0 R$_{25}$. The surface-density of dust declines with radius at a similar rate to the surface-density of stars but more slowly than the surface-density of the star-formation rate. Studies based on dust extinction and reddening of high-redshift quasars have concluded that there are substantial amounts of dust in intergalactic space. By combining our results with the number counts and angular correlation function from the SDSS, we show that with Milky Way type dust we can explain the reddening of the quasars by the dust within galactic disks alone. Given the uncertainties in the properties of any intergalactic dust, we cannot rule out its existence, but our results show that statistical investigations of the dust in galactic halos that use the reddening of high-redshift objects must take account of the dust in galactic disks.

Impact of galactic and intergalactic dust on the stellar EBL

Current theories assume that the low intensity of the stellar extragalactic background light (stellar EBL) is caused primarily by finite age of the Universe because the finite age limits the number of photons pumped into the space by galaxies and thus the sky is dark in the night. We oppose this opinion and show that two main factors are responsible for the extremely low intensity of the observed stellar EBL: (1) a low mean surface brightness of galaxies, which causes a low luminosity density in the local Universe, and (2) light extinction due to absorption by galactic and intergalactic dust. Dust produces a partial opacity of galaxies and of the Universe. The galactic opacity reduces the intensity of light from more distant background galaxies obscured by foreground galaxies. The effective extinction AV for light passing through a galaxy is 0.2 mag. This causes that distant background galaxies do not contribute to the EBL significantly. In addition, light of distant galaxies is dimmed due to absorption by intergalactic dust. Even a minute intergalactic opacity of 1x10^(-2) mag per Gpc is high enough to produce significant effects on the EBL. The absorbed starlight heats up the galactic and intergalactic dust and is further re-radiated at the IR, FIR and micro-wave spectrum. Assuming static infinite universe with no galactic and intergalactic dust, the stellar EBL should be as high as the surface brightness of stars. However, if dust is considered, the predicted stellar EBL is about 290 nWm^(-2)sr^(-1), which is only 5 times higher than the observed value. Hence, the presence of dust has higher impact on the EBL than currently assumed. In the expanding universe, the calculated value of the EBL is further decreased, because the obscuration effect and intergalactic absorption become more pronounced at high redshifts when the matter was concentrated at smaller volume than at present.

Thixotropic Phenomena in Water: Quantitative Indicators of Casimir-Magnetic Transformations from Vacuum Oscillations (Virtual Particles)

The ~1.5 × 10−20 J which is considered a universal quantity and is associated with the movement of protons in water also relates to the ratio of the magnetic moment of a proton divided by its unit charge, multiplied by viscosity and applied over the O-H distance. There is quantitative evidence that thixotropy, the “spontaneous” increased viscosity in water when undisturbed, originates from the transformation of virtual particles or vacuum oscillations to real states through conversion of Casimir-magnetic energies that involve the frequency of the neutral hydrogen line and the upper bound threshold value for intergalactic magnetic fields. The results indicate that ½ of a single electron orbit is real (particle) and the other ½ is virtual (wave). The matter equivalent per s for virtual-to-real states for electrons in 1 mL of water with a neutral pH is consistent with the numbers of protons (H+) and the measured range of molecules in the coherent domains for both width and duration of growth and is similar to widths of intergalactic dust grains from which planets and stars may condense. The de Broglie momentum for the lower boundary of the width of coherent domains multiplied by the fine structure velocity of an electron is concurrent with the quantum when one proton is being removed from another and when the upper boundary of the rest mass of a photon is transformed by the product of velocities for putative “entanglement” and light. Theoretical and experimental results indicate that components of thixotropy, such as specific domains of intercalated water molecules, could display excess correlations over very large distances. Because the energies of the universal quantity and water converge it may be a special conduit for discrete transformations from virtual to real states.

Combined constraints on intergalactic dust from quasar colours and the soft X-ray background

Unless properly corrected for, the existence of intergalactic dust will introduce a redshift-dependent magnitude offset to standard candle sources. This would lead to overestimated luminosity distances compared to a dust-free universe and bias the cosmological parameter estimation as derived from, e.g., Type Ia supernova observations. In this paper, we model the optical extinction and X-ray scattering properties of intergalactic dust grains to constrain the intergalactic opacity using a combined analysis of observed quasar colours and the soft X-ray background. Quasar colours effectively constrain the amount of intergalactic dust grains smaller than m, to the point where we expect the corresponding systematic error in the Type Ia supernova magnitude–redshift relation to be sub-dominant. Soft X-ray background observations are helpful in improving the constraints on very large dust grains for which the amount of optical reddening is very small and therefore is more difficult to correct for. Our current upper limit corresponds to ∼0.25 mag dimming at optical wavelengths for a source at redshift z = 1, which is too small to alleviate the need for dark energy but large in terms of relative error. However, we expect it to be possible to lower this bound considerably with an improved understanding of the possible sources of the X-ray background, in combination with observations of compact X-ray sources such as active galactic nuclei.

COSMOLOGICAL X-RAY SCATTERING FROM INTERGALACTIC DUST

High resolution X-ray imaging offers a unique opportunity to probe the nature of dust in the z ~< 2 universe. Dust grains 0.1- 1 um in size will scatter soft X-rays, producing a diffuse "halo" image around an X-ray point source, with a brightness ~ few % confined to an arcminute-sized region. We derive the formulae for scattering in a cosmological context and calculate the surface brightness of the scattering halo due to (i) an IGM uniformly enriched (Omega_ d ~ 10^-5) by a power-law distribution of grain sizes, and (ii) a DLA-type (N_H ~ 10^21 cm^-2) dust screen at cosmological distances. The morphology of the surface brightness profile can distinguish between the two scenarios above, place size constraints on dusty clumps, and constrain the homogeneity of the IGM. Thus X-ray scattering can gauge the relative contribution of the first stars, dwarf galaxies, and galactic outflows to the cosmic metallicity budget and cosmic history of dust. We show that, because the amount of intergalactic scattering is overestimated for photon energies < 1 keV, the non-detection of an X-ray scattering halo by Petric et al. (2006) is consistent with `grey' intergalactic dust grains (Omega_d ~ 10^-5$) when the data is restricted to the 1-8 keV band. We also calculate the systematic offset in magnitude, delta m ~ 0.01, for such a population of graphite grains, which would affect the type of supernova survey ideal for measuring dark energy parameters within ~ 1% precision.

FAR-INFRARED EMISSION FROM THE INTERGALACTIC MEDIUM IN STEPHAN’S QUINTET REVEALED BY AKARI

The Stephan's Quintet (SQ, HCG92) was observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared (IR) bands at 65, 90, 140, and 160 um. The AKARI four-band images of the SQ show far-IR emission in the intergalactic medium (IGM) of the SQ. In particular, the 160 um band image shows single peak emission in addition to the structure extending in the North-South direction along the shock ridge as seen in the 140 um band, H2 emission and X-ray emission. Whereas most of the far-IR emission in the shocked region comes from the cold dust component, shock-powered [CII]158um emission can significantly contribute to the emission in the 160 um band that shows a single peak at the shocked region. In the shocked region, the observed gas-to-dust mass ratio is in agreement with the Galactic one. The color temperature of the cold dust component (~20 K) is lower than that in surrounding galaxies (~30 K). We discuss a possible origin of the intergalactic dust emission.

Intergalactic dust extinction in hydrodynamic cosmological simulations

Recently Menard et al. detected a subtle but systematic change in the mean color of quasars as a function of their projected separation from foreground galaxies, extending to comoving separations of ~10Mpc/h, which they interpret as a signature of reddening by intergalactic dust. We present theoretical models of this remarkable observation, using SPH cosmological simulations of a (50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and assumes that dust traces the intergalactic metals. The predicted galaxy-dust correlation function is similar in form to the galaxy-mass correlation function, and reproducing the MSFR data requires a dust-to-metal mass ratio of 0.24, about half the value in the Galactic ISM. Roughly half of the reddening arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We also examine a simulation with no galactic winds, which predicts a much smaller fraction of intergalactic metals (3% vs. 35%) and therefore requires an unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The no-wind simulation can be reconciled with the data if we also allow reddening to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean visual extinction of A_V ~0.0133 mag out to z=0.5, with a sightline-to-sightline dispersion similar to the mean, which could be significant for future supernova cosmology studies. Reproducing the MSFR results in these simulations requires that a large fraction of ISM dust survive its expulsion from galaxies and its residence in the intergalactic medium. Future observational studies that provide higher precision and measure the dependence on galaxy type and environment will allow detailed tests for models of enriched galactic outflows and the survival of IG dust.

On the impact of intergalactic dust on cosmology with Type Ia supernovae

Supernova measurements have become a key ingredient in current determinations of cosmological parameters. These sources can however be used as standard candles only after correcting their apparent brightness for a number of effects. In this paper we discuss some limitations imposed by the formalism currently used for such corrections and investigate the impact on cosmological constraints. We show that color corrections are, in general, expected to be biased. In addition, color excesses which do not add a significant scatter to the observed SN brightnesses affect the value of cosmological parameters but leave the slope of the color-luminosity relation unchanged. We quantify these biases in the context of the redshift-dependent dust extinction suggested by the recent detection of intergalactic dust by Menard et al. (2009). Using a range of models for the opacity of the Universe as a function of redshift, we find that color-magnitude-stretch scaling relations are virtually insensitive to the presence of cosmic dust while cosmological parameters such as Omega_M and w are biased at the level of a few percent, i.e. offsets comparable to the current statistical errors. Future surveys will be able to limit the impact of intergalactic extinction by observing at larger wavelengths. In addition such datasets will provide direct detections of intergalactic dust by cross-correlating SN colors and the density of foreground galaxies, which can be used as a consistency check on the cosmic dust extinction correction. Alternatively, such biases could be avoided by correcting the colors of supernovae on an object-by-object basis with accurate photometry.

Intergalactic dust and its photoelectric heating

We have examined dust photoelectric heating in the intergalactic medium (IGM). The heating rate in a typical radiation field of the IGM is represented by Γpe=1.2x10-34 erg s-1 cm-3 (D/10-4)(nH/10-5 cm-3)43(JL/10-21 erg s-1 cm-2 HZ-1 sr-1)2/3(T104 K)-1/6), where D is the dust-to-gas mass ratio, nH is the hydrogen number density, JL is the mean intensity at the hydrogen Lyman limit of the background radiation, and T is the gas temperature, if we assume the new X-ray photoelectric yield model by Weingartner et al. (2006) and the dust size distribution in the Milky Way by Mathis et al. (1977). This heating rate dominates the HI and HeII photoionization heating rates when the hydrogen number density is less than ∼10-6 cm-3 if D = 10-4 which is 1% of that in the Milky Way, although the heating rate is a factor of 2–4 smaller than that with the old yield model by Weingartner and Draine (2001). The grain size distribution is very important. If only large (≥0.1 μm) grains exist in the IGM, the heating rate is reduced by a factor of ≏5. Since dust heating is more efficient in a lower density medium relative to the photoionization heating, it may cause an inverted temperature-density relation in the low-density IGM, as suggested by Bolton et al. (2008). Finally, we have found that dust heating is not very important in the mean IGM before the cosmic reionization.

Upper limit on dimming of cosmological sources by intergalactic grey dust from the soft X-ray background

Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft X-ray background (SXB) at photon energies 0.5<E<2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse X-ray halos around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains (with radii in the range a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our results are only weakly dependent on the grain size distribution or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.

COSMIC TRANSPARENCY: A TEST WITH THE BARYON ACOUSTIC FEATURE AND TYPE Ia SUPERNOVAE

Conservation of the phase-space density of photons plus Lorentz invariance requires that the cosmological luminosity distance be larger than the angular diameter distance by a factor of $(1+z)^2$, where $z$ is the redshift. Because this is a fundamental symmetry, this prediction--known sometimes as the "Etherington relation" or the "Tolman test"--is independent of world model, or even the assumptions of homogeneity and isotropy. It depends, however, on Lorentz invariance and transparency. Transparency can be affected by intergalactic dust or interactions between photons and the dark sector. Baryon acoustic feature and type Ia supernovae measures of the expansion history are differently sensitive to the angular diameter and luminosity distances and can therefore be used in conjunction to limit cosmic transparency. At the present day, the comparison only limits the change $\Delta\tau$ in the optical depth from redshift 0.20 to 0.35 at visible wavelengths to $\Delta\tau < 0.13$ at 95% confidence. In a model with a constant comoving number density $n$ of scatterers of constant proper cross-section $\sigma$, this limit implies $n \sigma< 2\times10^{-4} h \Mpc^{-1}$. These limits depend weakly on the cosmological world model. Within the next few years, the limits could extend to redshifts $z\approx2.5$ and improve to $n \sigma<1.1 \times10^{-5} h \Mpc^{-1}$. Cosmic variance will eventually limit the sensitivity of any test using the BAF at the $n \sigma\sim 4\times10^{-7} h \Mpc^{-1}$ level. Comparison with other measures of the transparency is provided; no other measure in the visible is as free of astrophysical assumptions.