f(R) modified gravity

Realistic simulations of galaxy formation in f(R) modified gravity

Nature Astronomy (2019)

Future astronomical surveys will gather information that will allow gravity to be tested on cosmological scales, where general relativity is currently poorly constrained. We present a set of cosmological hydrodynamical simulations that follow galaxy formation in f(R) modified gravity models and are dedicated to finding observational signatures to help distinguish general relativity from alternatives using this information. The simulations employ the IllustrisTNG model and a new modified gravity solver in AREPO, allowing the interplay of baryonic feedback and modified gravity to be studied in the same simulation, and the degeneracy between them in the matter power spectrum to be resolved. We find that the neutral hydrogen power spectrum is suppressed substantially in f(R) gravity, which allows this model to be constrained using upcoming data from the Square Kilometre Array. Disk galaxies can form in our f(R) gravity simulations, even in the partially screened regime, and their galaxy stellar properties are only mildly affected. We conclude that modified gravity allows the formation of realistic galaxies and leaves observable signatures on large scales.


Discovery of expanding cosmos

Move over, Hubble: Discovery of expanding cosmos assigned to little-known Belgian astronomer-priest

By Daniel Clery | Oct. 29, 2018 , 3:45 PM

Hubble’s Law, a cornerstone of cosmology that describes the expanding universe, should now be called the Hubble-Lemaître Law, following a vote by the members of the International Astronomical Union (IAU), the same organization that revoked Pluto’s status as a planet. The change is designed to redress the historical neglect of Georges Lemaître, a Belgian astronomer and priest who in 1927 discovered the expanding universe—which also suggests a big bang. Lemaître published his ideas 2 years before U.S. astronomer Edwin Hubble described his observations that galaxies farther from the Milky Way recede faster.

The final tally of the 4060 cast votes, announced today by IAU, was 78% in favor of the name change, 20% against, and 2% abstaining. But the vote was not without controversy, both in its execution and the historical facts it was based on. Helge Kragh, a historian of science at the Niels Bohr Institute in Copenhagen, calls the background notes presented to IAU members “bad history.” Others argue it is not IAU’s job to rename physical laws. “It’s bad practice to retroactively change history,” says Matthias Steinmetz of the Leibniz Institute for Astrophysics in Potsdam, Germany. “It never works.”

Piero Benvenuti of the University of Padua in Italy, who stepped down as IAU general secretary in August, proposed the change last year because, he says, “historically, it felt not right.” In 1927 Lemaître calculated a solution to Albert Einstein’s general relativity equations that indicated the universe could not be static but was instead expanding. He backed up that claim with a limited set of previously published measurements of the distances of galaxies and their velocities, calculated from their Doppler shifts. However, he published his results in French, in an obscure Belgian journal, and so they went largely unnoticed.

In 1929, Hubble published his own observations showing a linear relationship between velocity and distance for receding galaxies. It became known as Hubble’s Law. “Hubble was clearly involved, but was not the first,” says astronomer Michael Merrifield of the University of Nottingham in the United Kingdom. “He was good at selling his story.”

The text of the IAU resolution, circulated to members ahead of the vote, asserts that Hubble and Lemaître met in 1928, at an IAU general assembly in Leiden, the Netherlands—between the publication of their two papers—and “exchanged views” about the blockbuster theory. Kragh says that meeting “almost certainly didn’t take place” and that IAU’s statement “has no foundation in documented history.” Benvenuti counters that historians know from comments from Hubble’s assistant that he returned very excited from Leiden and began to gather more data. “Who else could have talked to Hubble about this problem but Lemaitre?” Benvenuti asks.

The resolution has also come under fire for confusing two different issues: the expansion of the universe and the distance-velocity relation for galaxies, which is also known as the Hubble constant. Hubble never claimed to have discovered cosmic expansion, but did do much of observing work to nail down how fast the universe was expanding. “If the law is about the empirical relationship, it should be Hubble’s Law,” Kragh says. “If it is about cosmic expansion, it should be Lemaître’s Law.”

Members have also criticized IAU over the way the vote was conducted. Traditionally, IAU resolutions are debated at general assemblies, once every 3 years, and decided by a show of hands of attending members. But such a straw poll led to the unpopular 2006 vote that reclassified Pluto as a dwarf planet. “The IAU got badly burned over the Pluto thing,” Merrifield says. As a result, IAU introduced the provision of having an online vote of the whole 11,000 membership.

In the case of Hubble’s Law, attendees at the August general assembly in Vienna were frustrated by a very short debate, followed by a straw poll (74% in favor of the name change). The IAU executive committee invited others to submit questions electronically and launched the online vote at the beginning of October. Merrifield says there was not enough time and opportunity for debate. “The IAU presented the issue as neat and tidy, but it is a much more murky and messy tale,” he says. He says several other researchers could have a claim because they were also working on cosmic expansion and galaxy motion at the time.

A final concern is whether IAU is within its rights to weigh in on historical affairs. “There is no mandate to name physical laws,” Steinmetz says. IAU has acknowledged this and is only recommending the use of the term Hubble-Lemaître Law. Will it catch on? “No, I don’t think so,” Kragh says. “Hubble Law is ingrained in the literature for most of a century.”

In any event, says Merrifield, “It doesn’t matter all that much, really.”

Hverken Hubble eller Lemaître målte en eneste galakses radialhastighed. Alle astronomer vidste, at de anvendte radialhastigheder var målt af

Vesto Melvin Slipher

Lemaître var teoretiker, ikke en observerende astronom. Når han i den omtalte “obskure” artikkel finder den samme værdi for H0 som Hubble to år senere skyldes det, at han var i besiddelse af en liste med Hubbles afstandsbestemmelser. Sammenhængen mellem radialhastigheder og afstande viste ikke entydigt en lineær relation. Den kunne godt være kvadratisk. Hubble mente ikke, at den lineære relation var et bevis på, at Universet ekspanderede.

Det er efterhånden blevet kontroversielt hver gang IAU vedtager en resulution vedrørende et historisk emne.

Jeg vil derfor skrive lidt om de to forskellige versioner af den naturvidenskabelige metode:

  1. Den induktive metode, som finder en oftest lineær sammenhæng mellem to observerede størrelser. En sådan sammenhæng kaldes en “lov”, hvis sandhedsværdi beror på, at sammenhængen skal kunne gentages ved nye målinger. Der er ikke tale om en årsag/virkning-sammenhæng.
  2. Den deduktive metode, som ud fra nogle få fysiske principper udleder en matematisk model for nogle fysiske størrelser ud fra nogle begyndelsesbetingelser.

Teknisk set hører Hubbles lov hjemme under den induktive metode, hvorimod ideen om Universets ekspansion hører hjemme under den deduktive metode. Den deduktive metode kaldes også en teori.

Nu er rene kosmologiske målinger ofte så usikre, at de ikke kan stå alene. De kombineres næsten altid med den deduktive metode i form af en teori.

Lemaître fandt, at et ekspanderende univers medfører en lineær relation mellem afstand og rødforskydning. Lemaître fortalte i 1927 Einstein om denne lineære relation. Einstein fandt, at ideen var en matematisk mulighed, men en fysisk afskyelig tanke.

Einstein lægger meget mere vægt på fysisk intuition end på astronomiske observationer. Han var således overbevist om, at de lokale naturlove var bestemt af de fjerne massers indflydelse. Et ekspanderende univers ville åbne op for variable naturlove. Denne samme frygt var baggrunden for den senere “steady state”-teori.

Einstein blev først modstræbende overbevist om Universets ekspansion i 1933.

Hubbles lov ville aldrig være blevet godtaget, hvis det ikke havde været for Lemaîtres teoretiske udledning af loven, samt beviset for, at Einsteins statiske univers er ustabilt.


Ginzburg-Landau Dark Energy

Ginzburg-Landau Theory of Dark Energy: A Framework to Study Both Temporal and Spatial Cosmological Tensions Simultaneously

Abdolali Banihashemi, Nima Khosravi, Amir H. Shirazi

A dark energy model (DE) is proposed based on Ginzburg-Landau theory of phase transition (GLT). This model, GLTofDE, surprisingly provides a framework to study not only temporal tensions in cosmology e.g. H0 tension but also spatial anomalies of CMB e.g. the hemispherical asymmetry, quadrupole-octopole alignment and its orthogonality to dipole simultaneously. In the mean field approximation of GLTofDE, the potential is broken spontaneously. We modeled this transition and showed that GLTofDE can resolve both the H0 tension and Lyman-α anomaly in a non-trivial way. According to a χ2-analysis the transition happens at zt=0.746+0.028-0.039 while H0=73.5±1.1 km/s/Mpc and Ωk=-0.196+0.049-0.033 which are consistent with the latest H(z) reconstructions. In addition, the GLTofDE proposes a framework to address the CMB anomalies when it is considered beyond the mean field approximation. In this regime existence of a long wavelength mode is a typical consequence which is named the Goldstone mode in the case of continuous symmetries. This mode, which is an automatic byproduct in GLTofDE, makes different directions of the sky see different cosmological constants. This means one side of the sky should be colder than the other side which can describe observed dipole in CMB. In addition between initial stochastic pattern and the final state with one long wavelength mode, we can observe smaller patches or protrusions of the biggest remaining patch in the simulation. Our simulations show these protrusions are few in numbers and will be evolved according to Alan-Cahn mechanism. These protrusions can give an additional effect on CMB which is the existence of aligned quadrupole-octopole mode and its direction should be orthogonal to the dipole direction. We conclude that GLTofDE is a very rich framework both theoretically and phenomenologically.


21cm absorption without CDM

Predictions for the sky-averaged depth of the 21cm absorption signal at high redshift in cosmologies with and without non-baryonic cold dark matter

Stacy McGaugh

We consider the 21-cm absorption signal expected at high redshift in cosmologies with and without non-baryonic cold dark matter. The expansion of the early universe decelerates strongly with dark matter, but approximately coasts without it. This results in a different path length across the epochs when absorption is expected, with the consequence that the absorption is predicted to be a factor of ∼ 2 greater without dark matter than with it. Observation of such a signal would motivate consideration of extended theories of gravity in lieu of dark matter.


Deficit of Dark Matter in NGC1052-DF2

A Deficit of Dark Matter from Jeans Modeling of the Ultra-diffuse Galaxy NGC 1052-DF2

The discovery of the ultra-diffuse galaxy NGC 1052-DF2 and its peculiar population of star clusters has raised new questions about the connections between galaxies and dark matter halos at the extremes of galaxy formation. In light of debates over the measured velocity dispersion of its star clusters and the associated mass estimate, we constrain mass models of DF2 using its observed kinematics with a range of priors on the halo mass. Models in which the galaxy obeys a standard stellar-halo mass relation are in tension with the data and also require a large central density core. Better fits are obtained when the halo mass is left free, even after accounting for increased model complexity. The dynamical mass-to-light ratio for our model with a weak prior on the halo mass is 1.7+0.7-0.5 M/L⊙,V, consistent with the stellar population estimate for DF2. We use tidal analysis to find that the low-mass models are consistent with the undisturbed isophotes of DF2. Finally we compare with Local Group dwarf galaxies and demonstrate that DF2 is an outlier in both its spatial extent and its relative dark matter deficit.


Isotropic non-Gaussian toy models

Isotropic non-Gaussian gNL-like toy models that reproduce the Cosmic Microwave Background anomalies

Recent observations of the Cosmic Microwave Background (CMB) have allowed claims for statistical anomalies in the behaviour of the CMB fluctuations to be made. Although the statistical significance of these remain only at the ∼(2-3)σ significance level, the fact that there are many different anomalies, several of which support a possible deviation from statistical isotropy, warrants the search for models affording a common mechanism to generate them. The goal of this paper is to investigate whether all these anomalies could originate from non-Gaussianity and to determine which properties such non-Gaussian models should have. We present a simple isotropic, non-Gaussian class of toy-models which can reproduce six heavily debated anomalies. We compare the presence of anomalies in simulated toy-model maps as well as Gaussian maps. We find that the following anomalies which are also found in Planck data, are commonly occuring in the toy-model maps: (1) Large scale hemispherical asymmetry (large scale dipolar modulation), (2) small scale hemispherical asymmetry (alignment of the spatial distribution of CMB power over all scales ℓ=[2,1500]) , (3) a strongly non-Gaussian hot or cold spot, (4) a low power spectrum amplitude for ℓ<30, including specifically (5) a low quadrupole and an unusual alignment between the quadrupole and the octopole, and (6) parity asymmetry of the lowest multipoles. We remark that this class of toy-models resembles models of primordial non-Gaussianity characterized by strongly scale-dependent gNL-like trispectra.

De senere mange års observationer af den kosmiske mikrobølge-baggrundsstråling (CMB) har tilladt påstande om statistiske anomalier i baggrundsstrålingens fluktuationer. Skønt den statistiske signifikans begrænser sig til et (2-3)σ-niveau, er det et faktum, at flere af disse anomalier støtter en mulig afvigelse fra statistisk isotropi. Disse anomalier definerer nogle faste retninger i rummet. Man er derfor på udkik efter en fælles fysisk mekanisme, som kan forklare alle anomalierne.

Det er artiklens formål at undersøge, om anomalierne kan skyldes en ikke-gaussisk fordeling af fluktuationerne, samt at bestemme, hvilke egenskaber sådanne ikke-gaussiske modeller må have. Forfatterne præsenterer en simpel isotrop, ikke-gaussisk klasse af legetøjsmodeller, som kan reproducere seks meget omdiskuterede anomalier.

Forfatterne er blevet inspireret af de ikke-liniære led i gravitationspotentialet, som forekommer i visse inflationsmodeller. De undersøger isotrope, men ikke-gaussiske modeller, hvori de ikke-gaussiske fluktioner er årsagen til den tilsyneladende afvigelse fra statistisk isotropi i de observerede data.

Inflationsmodeller kan have både 2.-ordens- og 3.-ordensled i gravitationspotentialet:

Φ(x) = ΦG(x) + fNLG2(x) – <ΦG2(x)>) + gNLΦG3(x)

ΦG(x) er den lineære gaussiske del af gravitationspotentialet. Baggrundsstrålingen har en kold plet, som kun kan frembringes af 3.-ordensleddet, så forfatterne retter opmærksomheden mod gNL-leddet. Forskerholdet bag Planck har imidlertid vist, et skalauafhængigt gNL-led ikke kan forklare anomalierne, så artiklen undersøger i stedet gNL-lignende legetøjsmodeller med en kraftig skalaafhængighed.

Forfatterne understreger, at artiklens formål ikke er at finde en fysisk model, som kan tilpasses de observerede data, men i stedet at bestemme, hvilke egenskaber en fysisk model må have.


Planck 2018 results. VI.

Planck 2018 results. VI. Cosmological parameters

We present cosmological parameter results from the final full-mission Planck measurements of the CMB anisotropies. We find good consistency with the standard spatially-flat 6-parameter ΛCDM cosmology having a power-law spectrum of adiabatic scalar perturbations (denoted “base-ΛCDM” in this paper), from polarization, temperature, and lensing, separately and in combination. A combined analysis gives dark matter density Ωch2 = 0.120±0.001, baryon density Ωbh2 = 0.0224±0.0001, scalar spectral index ns = 0.965±0.004, and optical depth τ = 0.054±0.007 (in this abstract we quote 68% confidence regions on measured parameters and 95% on upper limits). The angular acoustic scale is measured to 0.03% precision, with 100θ* = 1.0411±0.0003. These results are only weakly dependent on the cosmological model and remain stable, with somewhat increased errors, in many commonly considered extensions. Assuming the base-ΛCDM cosmology, the inferred late-Universe parameters are: Hubble constant H0 = (67.4±0.5) km/s/Mpc; matter density parameter Ωm = 0.315±0.007; and matter fluctuation amplitude σ8 = 0.811±0.006. We find no compelling evidence for extensions to the base-ΛCDM model. Combining with BAO we constrain the effective extra relativistic degrees of freedom to be Neff = 2.99±0.17, and the neutrino mass is tightly constrained to ∑ mν < 0.12 eV. The CMB spectra continue to prefer higher lensing amplitudes than predicted in base-ΛCDM at over 2σ, which pulls some parameters that affect the lensing amplitude away from the base-ΛCDM model; however, this is not supported by the lensing reconstruction or (in models that also change the background geometry) BAO data.

Forfatterne finder ikke nogen overbevisende evidens for nødvendigheden af en udvidelse af ΛCDM-modellen.


Star Formation from the 21cm signal

Constraints on Early Star Formation from the 21-cm Global Signal

The tentative detection by the EDGES experiment of a global 21-cm absorption trough centered at redshift 17 opens up the opportunity to study the birth of the first luminous sources, the intensity of radiation backgrounds at cosmic dawn, the thermal and ionization history of the young intergalactic medium. Here, we focus on the astrophysical implications of the Lyman-alpha photon field needed to couple the spin temperature to the kinetic temperature of the gas at these early epochs. Under the basic assumption that the 21-cm signal is activated by extremely metal-poor stellar systems, we show that the EDGES results are consistent with an extrapolation of the declining galaxy UV luminosity density measured at 4<z<9 by deep HST observations. A substantially enhanced star formation rate density or new exotic sources of UV photons are not required at the redshifts of the EDGES signal. The amount of ionizing radiation produced by the same stellar systems that induce Lyman-alpha coupling is significant, of order 0.5 Lyman-continuum photons per H-atom per 100 Myr. To keep hydrogen largely neutral and delay the reionization process consistently with recent Planck CMB results, mean escape fractions of f_esc < 20% are required at z>15.


Einstein’s Universe: trouble with Hubble

Cosmological structure formation in numerical relativity

We perform large-scale cosmological simulations that solve Einstein’s equations directly via numerical relativity. Starting with initial conditions sampled from the cosmic microwave background, we track the emergence of a cosmic web without the need for a background cosmology. We measure the backreaction of large-scale structure on the evolution of averaged quantities in a matter-dominated universe. We find negligible global backreaction in our simulations, with cosmological parameters Ωm = 1.005, ΩR = -1.2×10-8, and ΩQ + ΩL = -2.9×10-9. Sampling smaller scales, above the homogeneity scale of the Universe (100-180 h-1 Mpc), we find 2-3% variations in mean spatial curvature and backreaction.

Local versus global expansion rates in inhomogeneous cosmological simulations with numerical relativity

In a fully inhomogeneous, anisotropic cosmological simulation with numerical relativity, we find a local measurement of the Hubble parameter can be 1.2% larger than a global measurement, considering scales comparable to Type 1a supernova surveys. We find inhomogeneities cannot fully resolve the tension between the Riess(2018) and Planck(2016) measurements, however in extreme cases the tension can be reduced to 2.3% σ.


Two Twin Type Ia Supernovae

Significant Luminosity Differences of Two Twin Type Ia Supernovae

The Type Ia supernovae (SNe Ia) 2011by, hosted in NGC 3972, and 2011fe, hosted in M101, are optical “twins,” having almost identical optical light-curve shapes, colours, and near-maximum-brightness spectra. However, SN 2011fe had significantly more ultraviolet (UV; 1600 < λ < 2500 A) flux than SN 2011by before and at peak luminosity. Theory suggests that SNe Ia with higher progenitor metallicity should (1) have additional UV opacity near peak and thus lower UV flux; (2) have an essentially unchanged optical spectral-energy distribution; (3) have a similar optical light-curve shape; and (4) because of the excess neutrons, produce more stable Fe-group elements at the expense of radioactive 56Ni and thus have a lower peak luminosity. Foley & Kirshner (2013) suggested that the difference in UV flux between SNe 2011by and 2011fe was the result of their progenitors having significantly different metallicities. The SNe also had a large, but insignificant, difference between their peak absolute magnitudes (ΔMV, peak = 0.60 ± 0.36 mag), with SN 2011fe being more luminous. We present a new Cepheid-based distance to NGC 3972, significantly improving the precision of the distance measurement for SN 2011by. With these new data, we determine that the SNe have significantly different peak luminosities (ΔMV, peak = 0.335 ± 0.069 mag), corresponding to SN 2011fe having produced 38% more 56Ni than SN 2011by, and providing additional evidence for progenitor metallicity differences for these SNe. We discuss how progenitor metallicity differences can contribute to the intrinsic scatter for light-curve-shape-corrected SN luminosities, the use of “twin” SNe for measuring distances, and implications for using SNe Ia for constraining cosmological parameters.