Signal from age of the first stars

[BjarneModerator Super Nova]

Der er kommet en ny artikel om massen af de første stjerner, som traditionelt kaldes Population III. Det er en fortsættelse af Baades gamle klassifikation Pop I og Pop II. Den nye opdagelse af en kraftig absorption fra 21cm linien ved en rødforskydningen z = 17 viser, at spintemperaturen, dvs forholdet mellem antallet af hydrogenatomer med elektronspin parallelt med eller imod kernens spin, var lavere end forventet. Spintemperaturen har normalt samme værdi som hydrogenatomernes kinetiske temperatur. S. A. Wouthuysen viste i 1952, at det er Lyman-alfa fotoner med bølgelængden 1216 Å = 121.6 nm, som låser spintemperaturen til hydrogenatomernes kinetiske temperatur. De ultraviolette fotoner stammer fra massive stjerner, så forekomsten af denne absorption viser umiddelbart, at der må findes massive stjerner ved rødforskydningen z = 17. Men absorptionen var dybere end ventet. Dette betyder, at atomernes temperatur er lavere end ventet. Det mørke stofs partikler har en lavere temperatur end hydrogenatomerne, så disses temperatur kan sænkes, hvis der sker en spredning mellem mørke partikler og hydrogenkernerne. Artiklen har undersøgt, hvad denne ekstre køling betyder for massen af de første Population-III stjerner. Det viser sig, at de samme masser og tværsnit for de mørke partikler, som kan forklare den dybe 21cm-absorption også kan forklare forekomsten af mere massive Population-III stjerner, som kan levere de nødvendige Lyman-alfa fotoner.

Baryon-dark matter scattering and first star formation

The recent detection of the sky-averaged 21-cm cosmological signal indicates a stronger absorption than the maximum allowed value based on the standard model. One explanation for the required colder primordial gas is the energy transfer between the baryon and dark matter fluids due to non-gravitational scattering. Here, we explore the thermal evolution of primordial gas, collapsing to form Population III (Pop III) stars, when this energy transfer is included. Performing a series of one-zone calculations, we find that the evolution results in stars more massive than in the standard model, provided that the dark matter is described by the best-fit parameters inferred from the 21-cm observation. On the other hand, a significant part of the dark matter parameter space can be excluded by the requirement to form massive Pop III stars sufficiently early in cosmic history. Otherwise, the radiation background needed to bring about the strong Wouthuysen-Field coupling at z >~ 17, inferred to explain the 21-cm absorption feature, could not be builtup. Intriguingly, the independent constraint from the physics of first star formation at high densities points to a similarly narrow range in dark matter properties, compared to the conclusions from the 21-cm signal imprinted at low densities.

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Jeg har samlet lidt flere oplysninger på min Blog:
https://www.kosmologi.eu/wordpress/?p=3649

[BjarneModerator Super Nova]

21-cm Fluctuations from Charged Dark Matter

The epoch of the formation of the first stars, known as the cosmic dawn, has emerged as a new arena in the search for dark matter. In particular, the first claimed 21-cm detection exhibits a deeper global absorption feature than expected, which could be caused by a low baryonic temperature. This has been interpreted as a sign for electromagnetic interactions between baryons and dark matter. However, in order to remain consistent with the rest of cosmological observations, only part of the dark matter is allowed to be charged, and thus interactive. This hypothesis has a striking prediction: large temperature anisotropies sourced by the velocity-dependent cooling of the baryons. Here we compute, for the first time, the 21-cm fluctuations caused by a charged component of the dark matter, including both the pre- and post-recombination evolution of all fluids. We find that, for the same parameters that can explain the anomalous 21-cm absorption signal, any fraction f_dm of charged dark matter larger than 2 % would cause 21-cm fluctuations with pronounced acoustic oscillations, and with an amplitude above any other known effects. These fluctuations would be observable at high significance with interferometers such as LOFAR and HERA, thus providing an additional probe of dark matter at cosmic dawn.

 

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About HERA

The Hydrogen Epoch of Reionization Array (HERA) is a radio telescope dedicated to observing large scale structure during and prior to the epoch of reionization. HERA is a second generation instrument which combines efforts and lessons learned from the Murchison Widefield Array (MWA) and the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER). The array is a large grid of 14 meter (42 ft) diameter non-tracking dishes packed into a hexagonal grid 300 m (900ft) across. This substantial collecting area increase gives an order of magnitude more sensitivity than first generation instruments and be capable of robust statistical characterization and has the sensitivity to enable first images of large scale HI structure.

 

[BjarneModerator Super Nova]

Mørkt stof er først og fremmest nødvendig for at kunne forklare powerspektret for den kosmiske mikrobølgebaggrundsstråling (CBM). At ΛCDM-modellen var nødvendig var allerede blevet klar i midten af 1990’erne, selvom nogle få nostalgiske tilhængere af Einstein – de Sitter modellen holdt fast i en flad model med koldt mørkt stof uden en kosmologisk konstant. Opdagelsen af Universets positive acceleration i 1998 overbeviste de sidste skeptikere om eksistensen af “mørk” energi. Udtrykket blev opfundet af Michal Turner i afsky for den kosmologiske konstant.

Det man reelt observerer er den fysiske baryontæthed: Ω_bh² = 0.02227±0.00020
Den fysiske CDM tæthed: Ω_ch² = 0.1184±0.0012
En vinkeldiameter θ_*: 100θ_* = 1.04106±0.00041
h er hubblekonstanten i enheder af 100km/s/Mpc.

Man ser at de 3 størrelser er ganske godt bestemt. Alle andre størrelser udledes ud fra disse værdier under antagelse af, at ΛCDM er korrekt. Man har forgæves forsøgt at påvise en afvigelse fra den simple kosmologiske konstant Λ. Man ser også, at tætheden af det mørke stof er ganske godt bestemt. En alternativ teori skal kunne forklare dette faktum. Men disse målinger udelukker ikke, at en mindre del af det mørke stof består af en anden type partikler. Sådanne relativt få partikler kan måske forklare 21cm-målingerne ved z=17.

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Constraining Non-Cold Dark Matter Models with the Global 21-cm Signal

Any particle dark matter (DM) scenario featuring a suppressed power spectrum of astrophysical relevance results in a delay of galaxy formation. As a consequence, such scenarios can be constrained using the global 21-cm absorption signal initiated by the UV radiation of the first stars. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) recently reported the first detection of such an absorption signal at redshift ∼17. While its amplitude might indicate the need for new physics, we solely focus on the timing of the signal to test non-cold DM models. Assuming a conservative upper limit for star-formation based on radiation-hydrodynamics simulations, we are able to derive unprecedented constraints on a variety of non-cold DM models. For example, the mass of thermal warm DM is limited to m_TH > 6.1 keV, while mixed DM scenarios (featuring a cold and a hot component) are constrained to a hot DM fraction below 17 percent. The ultra-light axion DM model is limited to masses m_a > 8×10^-21 eV, a regime where its wave-like nature is pushed far below the kiloparsec scale. Finally, sterile neutrinos from resonant production can be fully disfavoured as a dominant DM candidate. The results of this paper show that the 21-cm absorption signal is a powerful discriminant of non-cold dark matter, allowing for significant improvements over to the strongest current limits. Confirming the result from EDGES is paramount in this context.

Denne nye artikel viser, at måling af det globale 21cm-signal er en vigtig metode til at udelukke alternative modellser for det mørke stof.

[BjarneModerator Super Nova]

Concerns about Modelling of Foregrounds and the 21-cm Signal in EDGES data

We have re-analyzed the data in which Bowman et al. (2018) identified a feature that could be due to cosmological 21-cm line absorption in the intergalactic medium at redshift z~17. If we use exactly their procedures then we find almost identical results, but the fits imply either non-physical properties for the ionosphere or unexpected structure in the spectrum of foreground emission (or both). Furthermore we find that making reasonable changes to the analysis process, e.g., altering the description of the foregrounds or changing the range of frequencies included in the analysis, gives markedly different results for the properties of the absorption profile. We can in fact get what appears to be a satisfactory fit to the data without any absorption feature if there is a periodic feature with an amplitude of ~0.05 K present in the data. We believe that this calls into question the interpretation of these data as an unambiguous detection of the cosmological 21-cm absorption signature.

Denne artikel åbner tvivl om den anvendte forgrundsmodel for radiostråling fra Mælkevejen og Jordens ionosfære. Der er almindelig enighed om, at Mælkevejens stråling kan beskrives ved 3 parametre, samt at ionosfærens stråling kan beskrives ved 2 parametre. EDGES-artiklen anvender en “fysisk motiveret” lineariseret formel med 5 parametre, så antallet af parametre er korrekt. Absorptionsprofilen beskrives i EDGES-artiklen ved 4 parametre. Det er det samlede fit af disse 9 parametre, der giver detektionen af 21cm-absorptionen. Denne artikel gør imidlertid opmærksom på, at 2 af de 5 fittede forgrundsparametre svarer til en ufysisk ionosfære med en negativ optisk dybde! Det er kombinationen af disse 2 ufysiske parametre og de 4 profilparametre, som giver det gode fit. Nuvel, det er tilladt at anvende en fitteformel; men man burde ikke kalde den “fysisk motiveret”. Artiklen gør endvidere opmærksom på, at de rå data indeholder en mystisk peridicitet med perioden 12 MHz. Denne priodiske variation kan fittes ved en kombination af en flad absorptionsprofil og den angivne formel for forgrunden. EDGES-forfatterne har gjort sig store anstrengelser for at finde en forklaring på variationen, men uden held. Den nye artikel mener, at man ikke har en sikker detektion før en instrumentel effekt er udelukket.

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