Cosmic Explosions, Life in the Universe, and the Cosmological Constant

Gamma-ray bursts (GRBs) are copious sources of gamma rays whose interaction with a planetary atmosphere can pose a threat to complex life. Using recent determinations of their rate and probability of causing massive extinction, we explore what types of universes are most likely to harbor advanced forms of life. We use cosmological $N$-body simulations to determine at what time and for what value of the cosmological constant ($\mathrm{\Lambda }$) the chances of life being unaffected by cosmic explosions are maximized. Life survival to GRBs favors Lambda-dominated universes. Within a cold dark matter model with a cosmological constant, the likelihood of life survival to GRBs is governed by the value of $\mathrm{\Lambda }$ and the age of the Universe. We find that we seem to live in a favorable point in this parameter space that minimizes the exposure to cosmic explosions, yet maximizes the number of main sequence (hydrogen-burning) stars around which advanced life forms can exist.

Figure 1

Number of isolated halos as a function of redshift that are protected from the damaging effect on life of GRBs; we show four cosmologies and two isolated radii of satellites: 20 kpc (upper panel) and 50 kpc (lower panel). To guide the eye, dashed lines show extrapolations to $z=0$ due to limitations in remapping the Millennium-ii simulation, as this would have needed it to be run into the future. The number of life-protected regions is significantly larger in the $\mathrm{\Lambda }$-dominated cosmologies. This is due to the effect of $\mathrm{\Lambda }$ at late times at clearing and smoothing virialized regions. Also remarkable is the fact that only below $z\approx 1$, the number of isolated halos grows significantly. This is about 7 Gyr ago, not too dissimilar from the age of Earth.

Figure 2

The probability distribution function for ${\rho }_{\mathrm{\Lambda }}$ normalized to the probability for the Einstein–de Sitter (EdS, $\mathrm{\Lambda }=0$) case. The solid (dashed) line corresponds to exclusion regions of 20 (50) kpc and result from this work. For high $\mathrm{\Lambda }$ values, the exponential suppression of Ref. [31] takes over. This is indicated by the shaded regions. The arrow shows the value of ${\rho }_{\mathrm{\Lambda }}$ for the concordance $\mathrm{\Lambda }\mathrm{CDM}$ model. Our result suppresses the probability of low values of $\mathrm{\Lambda }$ previously allowed and, in fact, favored, by the argument in Refs. [1, 2].