Charles H. Lineweaver

The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way

We modeled the evolution of the Milky Way Galaxy to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun.

How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars

Sun-like stars have stellar, brown dwarf, and planetary companions. To help constrain their formation and migration scenarios, we analyze the close companions (orbital period <5 yr) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf, and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost 2 orders of magnitude from 1 M☉ stellar companions to the brown dwarf desert and then rises by more than an order of magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the planetary and stellar companion mass functions are of opposite sign and are incompatible at the 3 σ level, thus yielding a brown dwarf desert. The minimum number of companions per unit interval in log mass (the driest part of the desert) is at M = 31MJ. Approximately 16% of Sun-like stars have close (P < 5 yr) companions more massive than Jupiter: 11% ± 3% are stellar, <1% are brown dwarf, and 5% ± 2% are giant planets. The steep decline in the number of companions in the brown dwarf regime, compared to the initial mass function of individual stars and free-floating brown dwarfs, suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.

An estimate of the age distribution of terrestrial planets in the universe: quantifying metallicity as a selection effect

Abstract Planets such as the Earth cannot form unless elements heavier than helium are available. These heavy elements, or “metals”, were not produced in the Big Bang. They result from fusion inside stars and have been gradually building up over the lifetime of the Universe. Recent observations indicate that the presence of giant extrasolar planets at small distances from their host stars is strongly correlated with high metallicity of the host stars. The presence of these close-orbiting giants is incompatible with the existence of Earth-like planets. Thus, there may be a Goldilocks selection effect: with too little metallicity, Earths are unable to form for lack of material; with too much metallicity, giant planets destroy Earths. Here I quantify these effects and obtain the probability, as a function of metallicity, for a stellar system to harbor an Earth-like planet. I combine this probability with current estimates of the star formation rate and of the gradual buildup of metals in the Universe to obtain an estimate of the age distribution of Earth-like planets in the Universe. The analysis done here indicates that three-quarters of the Earth-like planets in the Universe are older than the Earth and that their average age is 1.8±0.9 billion years older than the Earth. If life forms readily on Earth-like planets—as suggested by the rapid appearance of life on Earth—this analysis gives us an age distribution for life on such planets and a rare clue about how we compare to other life which may inhabit the Universe.

Signatures of a shadow biosphere.

Astrobiologists are aware that extraterrestrial life might differ from known life, and considerable thought has been given to possible signatures associated with weird forms of life on other planets. So far, however, very little attention has been paid to the possibility that our own planet might also host communities of weird life. If life arises readily in Earth-like conditions, as many astrobiologists contend, then it may well have formed many times on Earth itself, which raises the question whether one or more shadow biospheres have existed in the past or still exist today. In this paper, we discuss possible signatures of weird life and outline some simple strategies for seeking evidence of a shadow biosphere.

A larger estimate of the entropy of the universe

Using recent measurements of the supermassive black hole (SMBH) mass function, we find that SMBHs are the largest contributor to the entropy of the observable universe, contributing at least an order of magnitude more entropy than previously estimated. The total entropy of the observable universe is correspondingly higher, and is S obs = 3.1+3.0 –1.7 × 10104 k. We calculate the entropy of the current cosmic event horizon to be S CEH = 2.6 ± 0.3 × 10122 k, dwarfing the entropy of its interior, S CEH int = 1.2+1.1 –0.7 × 10103 k. We make the first tentative estimate of the entropy of weakly interacting massive particle dark matter within the observable universe, S dm = 1088 ± 1 k. We highlight several caveats pertaining to these estimates and make recommendations for future work.

What Fraction of Sun-like Stars Have Planets?

The radial velocities of ~1800 nearby Sun-like stars are currently being monitored by eight high-sensitivity Doppler exoplanet surveys. Approximately 90 of these stars have been found to host exoplanets massive enough to be detectable. Thus, at least ~5% of target stars possess planets. If we limit our analysis to target stars that have been monitored the longest (~15 years), ~11% possess planets. If we limit our analysis to stars monitored the longest and whose low surface activity allows the most precise velocity measurements, ~25% possess planets. By identifying trends of the exoplanet mass and period distributions in a subsample of exoplanets less biased by selection effects and linearly extrapolating these trends into regions of parameter space that have not yet been completely sampled, we find that at least ~9% of Sun-like stars have planets in the mass and orbital period ranges M sin i > 0.3MJup and P 0.1MJup and P < 60 years. Even this larger area of the log(mass)-log(period) plane is less than 20% of the area occupied by our planetary system, suggesting that this estimate is still a lower limit to the true fraction of Sun-like stars with planets, which may be as large as ~100%.

Does the Rapid Appearance of Life on Earth Suggest that Life is Common in the Universe

It is sometimes assumed that the rapidity of biogenesis on Earth suggests that life is common in the Universe. Here we critically examine the assumptions inherent in this if-life-evolved-rapidly-life-must-be-common argument. We use the observational constraints on the rapidity of biogenesis on Earth to infer the probability of biogenesis on terrestrial planets with the same unknown probability of biogenesis as the Earth. We find that on such planets, older than approximately 1 Gyr, the probability of biogenesis is > 13% at the 95% confidence level. This quantifies an important term in the Drake Equation but does not necessarily mean that life is common in the Universe.

Applications of Wavelets to the Analysis of Cosmic Microwave Background Maps

We consider wavelets as a tool to perform a variety of tasks in the context of analyzing cosmic microwave background (CMB) maps. Using Spherical Haar Wavelets we define a position and angular-scale-dependent measure of power that can be used to assess the existence of spatial structure. We apply planar Daubechies wavelets for the identification and removal of points sources from small sections of sky maps. Our technique can successfully identify virtually all point sources which are above 3� and more than 80% of those above 1�. We discuss the trade-offs between the levels of correct and false detections. We denoise and compress a 100,000 pixel CMB map by a factor of � 10 in 5 seconds achieving a noise reduction of about 35%. In contrast to Wiener filtering the compression process is model independent and very fast. We discuss the usefulness of wavelets for power spectrum and cosmological parameter estimation. We conclude that at present wavelet functions are most suitable for identifying localized sources.

What Can Cosmic Microwave Background Observations Already Say about Cosmological Parameters in Open and Critical-Density Cold Dark Matter Models?

We use a combination of the most recent cosmic microwave background (CMB) flat-band power measurements to place constraints on Hubbles constant h and the total density of the universe ?0 in the context of inflation-based cold dark matter (CDM) models with no cosmological constant. We use ?2 minimization to explore the four-dimensional parameter space having as free parameters, h, ?0, the power-spectrum slope n, and the power-spectrum normalization at ? = 10. Conditioning on ?0 = 1, we obtain h = 0.33 ? 0.08. Allowing ?0 to be a free parameter reduces the ability of the CMB data to constrain h, and we obtain 0.26 0.53. A strong correlation between acceptable h and ?0 values leads to a new constraint ?0h1/2 = 0.55 ? 0.10. We quote ??2 = 1 contours as error bars; however, because of nonlinearities of the models, these may be only crude approximations to 1 ? confidence limits. A favored open model with ?0 = 0.3 and h = 0.70 is more than ~4 ? from the CMB data best-fit model and is rejected by goodness-of-fit statistics at the 99% confidence level. High baryonic models (?bh2 ~ 0.026) yield the best CMB ?2 fits and are more consistent with other cosmological constraints. The best-fit model has n = 0.91 -->+ 0.29?0.09 and Q10 = 18.0 -->+ 1.2?1.5 ?K. Conditioning on n = 1, we obtain h = 0.55 -->+ 0.13?0.19, ?0 = 0.70 with a lower limit ?0 > 0.58, and Q10 = 18.0 -->+ 1.4?1.5 ?K. The amplitude and position of the dominant peak in the best-fit power spectrum are Apeak = 76 -->+ 3?7 ?K and ?peak = 260 -->+ 30?20. Unlike the ?0 = 1 case we considered previously, CMB h results are now consistent with the higher values favored by local measurements of h but only if 0.55 ?0 0.85. Using an approximate joint likelihood to combine our CMB constraint on ?0h1/2 with other cosmological constraints, we obtain h = 0.58 ? 0.11 and ?0 = 0.65 -->+ 0.16?0.15.

Black hole versus cosmological horizon entropy

The generalized second law of thermodynamics states that entropy always increases when all event horizons are attributed with an entropy proportional to their area. We test the generalized second law by investigating the change in entropy when dust, radiation and black holes cross a cosmological event horizon. We generalize for flat, open and closed Friedmann–Robertson–Walker universes by using numerical calculations to determine the cosmological horizon evolution. In most cases, the loss of entropy from within the cosmological horizon is more than balanced by an increase in cosmological event horizon entropy, maintaining the validity of the generalized second law of thermodynamics. However, an intriguing set of open universe models shows an apparent entropy decrease when black holes disappear over the cosmological event horizon. We anticipate that this apparent violation of the generalized second law will disappear when solutions are available for black holes embedded in arbitrary backgrounds.