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As the Sun moves through the surrounding partially ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge exchange with the supersonic solar wind, create a population of hot pick-up ions (PUIs). Until recently, the consensus was that the shape of the heliosphere is comet-like. The termination shock crossing by Voyager 2 demonstrated that the heliosheath (the region of shocked solar wind) pressure is dominated by PUIs; however, the impact of the PUIs on the global structure of the heliosphere has not been explored. Here we use a novel magnetohydrodynamic model that treats the PUIs as a separate fluid from the thermal component of the solar wind. The depletion of PUIs, due to charge exchange with the neutral hydrogen atoms of the interstellar medium in the heliosheath, cools the heliosphere, ‘deflating’ it and leading to a narrower heliosheath and a smaller and rounder shape, confirming the shape suggested by Cassini observations. The new model reproduces both the properties of the PUIs, based on the New Horizons observations, and the solar wind ions, based on the Voyager 2 spacecraft observations as well as the solar-like magnetic field data outside the heliosphere at Voyager 1 and Voyager 2. A magnetohydrodynamic model of the interaction between the solar wind and the interstellar medium shows that our heliosphere has a round shape, not a tail-like shape as usually assumed. The model reproduces the observations from Cassini, New Horizons and the two Voyager probes.

It is commonly argued that an undesirable feature of a theoretical or phenomenological model is that salient observables are sensitive to values of parameters in the model. But in what sense is it undesirable to have such ‘fine-tuning’ of observables (and hence of the underlying model)? In this paper, we argue that the fine-tuning can be interpreted as a shortcoming of the explanatory capacity of the model: in particular it signals a lack of a particular type of explanatory depth. The aspect of depth that we probe relates most closely to a lack of sensitivity to changes in parameters associated with such models. In support of this argument, we develop a schema—for (a certain class of) models that arise broadly in physical settings—that quantitatively relates fine-tuning of observables to a lack of depth of explanations based on these models. We apply our schema in two different settings in which, within each setting, we compare the depth of two competing explanations. The first setting involves explanations for the Euclidean nature of spatial slices of the universe today: in particular, we compare an explanation provided by the big-bang model of the early 1970s (where no inflationary period is included) with an explanation provided by a general model of cosmic inflation. The second setting has a more phenomenological character, where the goal is to infer from a limited sequence of data points, using maximum entropy techniques, the underlying probability distribution from which these data are drawn. In both of these settings we find that our analysis favors the model that intuitively provides the deeper explanation of the observable(s) of interest. We thus provide an account that relates two ‘theoretical virtues’ of models used broadly in physical settings—namely, a lack of fine-tuning and explanatory depth—and argue that finely tuned models sacrifice explanatory depth.

The interaction between the outflow of gas from a quasar and the interstellar medium can boost protons to relativistic energies. Collisions between such protons can explain a significant fraction of the unexplained extragalactic gamma-ray background. The origin of the extragalactic γ-ray background permeating throughout the Universe remains a mystery forty years after its discovery1. The extrapolated population of blazars can account for only half of the background radiation in the energy range of ∼0.1–10 GeV (refs 2,3). Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic γ-ray background and naturally match its spectral fingerprint, with a generic break above ∼1 GeV. The associated γ-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi-LAT observations of the extragalactic γ-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows4,5,6,7 and theoretical predictions8,9. Although our model explains the data, there might be additional contributing sources.

Any ambitious construction project requires architects and engineers. As research shifts towards large groups that focus on the engineering aspects of linking data to existing models, architectural skills are becoming rare among young theorists.

Nature 542, 203–205 (2017); doi:10.1038/nature21361 After the completion of the initial work and submission of this Letter in 2015, some additional data shared with us in 2016 (that we were given to understand will be published shortly) were later incorporated into the analysis owing to an oversightand miscommunication.

A large number of faint galaxies, born less than a billion years after the Big Bang, have recently been discovered. Fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of megaparsecs, as observed along the lines of sight to the earliest known quasars. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized, leading to a drop in the cosmic star-formation rate. Here we report evidence for this suppression. We show that the post-reionization galaxies that produced most of the ionizing radiation at a redshift z approximately 5.5 must have had a mass in excess of approximately 10(10.9 +/- 0.5) solar masses (M(o)) or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (approximately 10(8) M(o)). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, before the epoch of dwarf galaxy suppression.

The first galaxies to appear in the Universe at redshifts z > 20 created ionized bubbles in the intergalactic medium of neutral hydrogen left over from the Big Bang. The ionized bubbles grew with time, surrounding clusters of dwarf galaxies and eventually overlapped quickly throughout the Universe over a narrow redshift interval near z approximately 6. This event signalled the end of the reionization epoch when the Universe was a billion years old. Measuring the size distribution of the bubbles at their final overlap phase is a focus of forthcoming programmes to observe highly redshifted radio emission from atomic hydrogen. Here we show that the combined constraints of cosmic variance and light travel time imply an observed bubble size at the end of the overlap epoch of approximately 10 physical Mpc, and a scatter in the observed redshift of overlap along different lines-of-sight of approximately 0.15. This scatter is consistent with observational constraints from recent spectroscopic data on the farthest known quasars. This implies that future radio experiments should be tuned to a characteristic angular scale of 0.5 degrees and have a minimum frequency bandwidth of approximately 8 MHz for an optimal detection of 21-cm flux fluctuations near the end of reionization.

The fraction of ionized hydrogen left over from the Big Bang provides evidence for the time of formation of the first stars and quasar black holes in the early Universe; such objects provide the high-energy photons necessary to ionize hydrogen. Spectra of the two most distant known quasars show nearly complete absorption of photons with wavelengths shorter than the Lyman alpha transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift of z approximately 6.3, about one billion years after the Big Bang. Here we show that the IGM surrounding these quasars had a neutral hydrogen fraction of tens of per cent before the quasar activity started, much higher than the previous lower limits of approximately 0.1 per cent. Our results, when combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination therefore suggest the presence of a second peak in the mean ionization history of the Universe.

The origin of the highest energy cosmic rays remains an enigma. They offer a window to new physics, including tests of physical laws relevant to their propagation and interactions, at energies unattainable by terrestrial accelerators. They must be accelerated locally, as otherwise background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Nearby Gamma Ray Bursts (GRBs), Hypernovae, Active Galactic Nuclei (AGNs) and their flares, have all been suggested and debated as possible sources. A local sub-population of type Ibc supernovae (SNe) with mildly relativistic outflows have been detected as sub-energetic GRBs or X-Ray Flashes (XRFs) and recently as radio afterglows without detected GRB counterparts. We measure the size-magnetic field evolution, baryon loading and energetics, using the observed radio spectra of SN 2009bb. We place such engine-driven SNe above the Hillas line and establish that they can readily explain the post-GZK UHECRs., 23 pages, 1 figure, version submitted to Nature Communications, embargoed from discussion in the press

Discoveries in astronomy — or, in fact, any branch of science — can only happen when people are open-minded and willing to take risks.

When and how the first stars and galaxies ionized the primordial hydrogen atoms that filled the early Universe is not known. Observations with a single radio antenna are opening a new window on the process. See Letter p.796

Too many young physicists embark on projects without knowing the risks. There is a better way, argues Abraham Loeb.

More young scientists should dedicate a portion of their attention to high-risk research despite the potential downside, argues Abraham Loeb.