Signal from age of the first stars

Fora ASTRO-FORUM NYT FRA VIDENSKABEN Signal from age of the first stars

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  • #316623

    Bjarne
    Moderator
      • Super Nova

      Signal from age of the first stars could shake up search for dark matter

      EDGES Science explained

      By Adrian Cho | Feb. 28, 2018 , 1:00 PM

      Using radio antennas the size of coffee tables, a small team of astronomers has glimpsed the cosmic dawn, the moment billions of years ago when the universe’s first stars began to shine. The observation also serves up surprising evidence that particles of dark matter—the unseen stuff that makes up most of the universe’s matter—may be much lighter than physicists thought.

      If it holds up, the result could sharpen cosmologists’ picture of the early universe and shake up the search for dark matter. “It’s going to generate a huge amount of interest,” says Kevork Abazajian, a theoretical cosmologist at the University of California (UC), Irvine. But others worry that the subtle radio signal reported by the team could be an artifact. “I don’t think that right now, at least in my mind, it’s a clear discovery,” says Aaron Parsons, an experimental cosmologist at UC Berkeley.

      The data come from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), a $2 million array of three radio antennas in the outback of Western Australia. The five EDGES researchers searched for signs that the hydrogen atoms that pervaded the newborn universe had absorbed microwaves lingering from the big bang.

      The absorption marks the moment just after the first stars began to shine. Before that moment, the atoms’ internal states were in equilibrium with the microwaves, emitting as much radiation as they absorbed. But light from the first stars jostled the atoms’ innards, disrupting the equilibrium and enabling the atoms to absorb more of the microwaves than they emit.

      The expansion of the universe stretches the absorption signal from its original 21-centimeter wavelength to longer radio wavelengths. However, radio noise from our galaxy is 30,000 times more intense. To subtract it, EDGES researchers relied on the noise’s smooth, precisely predictable spectrum. This week in Nature, they report detecting the tiny absorption signal—the cumulative shadows, they conclude, of hydrogen clouds that existed between 180 million and 250 million years after the big bang.

      It’s the first thing scientists have seen in the time between the cosmic microwave background, 380,000 years after the big bang, and the oldest known galaxy, which shone 400 million years later, says EDGES leader Judd Bowman. This is really the only possible probe that we have of the time before the stars, says Bowman, who is an experimental astrophysicist at Arizona State University in Tempe. Ultimately, scientists hope to use the absorption signal or the fainter emission of 21-centimeter radiation from gas clouds at slightly later times to map the 3D distribution of hydrogen during these so-called cosmic dark ages, tracing its evolution into embryonic galaxies.

      The absorption is more than twice as strong as predicted, which suggests that the hydrogen was significantly colder than previously thought. The gas must have lost heat to something even colder, and the only colder thing around was dark matter, which was coalescing into the clumps that would seed the formation of galaxies, reasons Rennan Barkana, an astrophysicist at Tel Aviv University in Israel. In a second paper in Nature, Barkana argues that to cool the hydrogen, the dark matter particles must have been less than five times as massive as a hydrogen atom. Otherwise the atoms would have bounced off them without losing energy and getting colder, just as a Ping-Pong ball will bounce off a bowling ball without slowing down.

      Many dark matter searches have targeted hypothetical weakly interacting massive particles, which are generally expected to weigh hundreds of times as much as a hydrogen atom. As those searches have come up empty, some physicists have begun searching for lighter dark matter particles. The new result may encourage them, Abazajian says.

      However, it’s too early to rule out a more mundane explanation for the unexpectedly strong absorption, cautions Katherine Freese, an astrophysicist at the University of Michigan in Ann Arbor. “Is [this scenario] the only way to explain this? Of course not.”

      A more pressing question is whether the signal is an experimental artifact, Parsons says. The measurements rely on calibrations that could produce false signals if they are off by just a few hundredths of a percent, he says. Bowman says he and his colleagues “have gone as far as we can go to ensure that there isn’t an error, but, of course, we’re eager for others to confirm the result.”

      Confirmation could come from other experiments that are probing the dark ages. Parsons leads one, called the Hydrogen Epoch of Reionization Array in South Africa, which is trying not just to detect the faint signals, but to map them across the sky. They may soon show whether cosmic dawn has really broken.

      #316624

      Bjarne
      Moderator
        • Super Nova

        Her er så nyheden i Nature:

        Astronomers detect light from the Universe’s first stars

        Der er også en video, som forklarer opdagelsen.

        Der er også en artikel, som forklarer, at de mørke partiklers masse er mindre end forventet for WIMPS, så de kan køle hydrogenatomerne:

        A surprising chill before the cosmic dawn

        An experiment to estimate when stars began to form in the Universe suggests that gas temperatures just before stars appeared had fallen well below predicted limits, and that dark matter is not as shadowy as was thought.

         

        #316631

        holger.nielsen
        Deltager
          • Asteroid

          Det lyder spændende, hvis man omsider er kommet på sporet af partiklerne hørende til mørkt stof.

          Men jeg husker stadig de begejstrede meldinger for et par år siden fra et mikrobølgeobservatorium på Sydpolen,

          hvor man meddelte at have fundet aftryk af gravitationsbølger i den kosmiske baggrundsstråling. Signalet viste sig senere at være falsk.

          “Abwarten und Tee trinken”.

          Holger Nielsen

          #316638

          Bjarne
          Moderator
            • Super Nova

            Kun nye observationer kan afgøre sagen. Men der er flere forskellige observationer, som afviger fra den “klassiske” forestilling om koldt mørkt stof. Man har længe vidst, at dværggalakser omkring Mælkevejen og Andromedagalaksen befinder sig i bestemte planer. Amplituden sigma8 af den lineære strukturdannelse er mindre end forventet. Der er forskellige forklaringer som f.eks., at der er afvigelser fra Einsteins generelle relativitetsteori. Det vil være af stor betydning, hvis man får en antydning af, hvad man skal lede efter. Her er en hurtig artikel om emnet:

            Insights on Dark Matter from Hydrogen during Cosmic Dawn

            The origin and composition of the cosmological dark matter remain a mystery. However, upcoming 21-cm measurements during cosmic dawn, the period of the first stellar formation, can provide new clues on the nature of dark matter. During this era, the baryon-dark matter fluid is the slowest it will ever be, making it ideal to search for dark matter elastically scattering with baryons through massless mediators, such as the photon. Here we explore whether dark-matter particles with an electric “minicharge” can significantly alter the baryonic temperature and, thus, affect 21-cm observations. We find that the entirety of the dark matter cannot be minicharged at a significant level, lest it interferes with Galactic and extragalactic magnetic fields. However, if minicharged particles comprise a subpercent fraction of the dark matter, and have charges ε ∼ 10-6—in units of the electron charge—and masses mχ ∼ 1-60 MeV, they can significantly cool down the baryonic fluid, and be discovered in 21-cm experiments. We show how this scenario can explain the recent result by the EDGES collaboration, which requires a lower baryonic temperature than possible within the standard model, while remaining consistent with all current observations.

            #316639

            Bjarne
            Moderator
              • Super Nova

              Her er en ny artikel, som understreger eksistensen af flere forklaringer på skivegalaksers rotationskurver.

              Theoretical implications of the galactic radial acceleration relation of McGaugh, Lelli, and Schombert

              Velocities in stable circular orbits about galaxies, a measure of centripetal gravitation, exceed the expected Kepler/Newton velocity as orbital radius increases. Standard LCDM attributes this anomaly to galactic dark matter. McGaugh et al have recently shown for 153 disc galaxies that observed radial acceleration is an apparently universal function of classical acceleration computed for observed galactic baryonic mass density. This is consistent with the empirical MOND model, not requiring dark matter. It is shown here that suitably constrained LCDM and conformal gravity (CG) also produce such a universal correlation function. LCDM requires a very specific dark matter distribution, while the implied CG nonclassical acceleration must be independent of galactic mass. All three constrained radial acceleration functions agree with the empirical baryonic v4 Tully-Fisher relation. Accurate rotation data in the nominally flat velocity range could distinguish between MOND, LCDM, and conformal gravity.

               

              #316640

              Bjarne
              Moderator
                • Super Nova

                Og her er en artikel om, hvordan hypotesen om ekstra køling af hydrogenatomer kan testes ved anvendelse af interferometre.

                Constraining Baryon–Dark Matter Scattering with the Cosmic Dawn 21-cm Signal

                The recent detection of an anomalously strong 21-cm signal of neutral hydrogen from Cosmic Dawn by the EDGES Low-Band radio experiment can be explained if cold dark matter particles scattered off the baryons draining excess energy from the gas. In this Letter we explore the expanded range of the 21-cm signal that is opened up by this interaction, varying the astrophysical parameters as well as the properties of dark matter particles in the widest possible range. We identify models consistent with current data by comparing to both the detection in the Low-Band and the upper limits from the EDGES High-Band antenna. We find that consistent models predict a 21-cm fluctuation during Cosmic Dawn that is between 3 and 30 times larger than the largest previously expected without dark matter scattering. The expected power spectrum exhibits strong Baryon Acoustic Oscillations imprinted by the velocity-dependent cross-section. The latter signature is a smoking gun of the velocity-dependent scattering and could be used by interferometers to verify the dark matter explanation of the EDGES detection.

                 

                #316652

                Bjarne
                Moderator
                  • Super Nova

                  _
                  Figure 1 | Temperature changes during the evolution of the Universe. The first two phases of the Universe were the ‘dark age’, before stars formed (grey), and the cosmic dawn (yellow), when clouds of hydrogen in early structures collapsed to form stars. The temperature of radiation (TR) left over from the Big Bang has declined slowly over time. The spin temperature (TS) of hydrogen that has not formed stars reflects the excitation state of the hydrogen atoms (solid blue line shows previous estimates of TS based on models). Bowman et al. use observations to estimate TS, and find that it dropped to lower values (red solid line) than predicted by models. Barkana proposes that this could be evidence for a previously unrecognized, non-gravitational interaction between normal and dark matter. Such interactions would mean that the ‘kinetic’ temperature of gas (TG) in the Universe also dropped to a lower minimum (red dashed line) than is predicted by known physics (blue dashed line).

                  The first stars to form generated copious fluxes of ultraviolet radiation that suffused the early Universe — a phenomenon referred to as the cosmic dawn. Many calculations have been performed to estimate when this occurred1, but no data-driven constraints on the timing have been available. In a paper in Nature, Bowman et al. report what might be the first detection of the thermal footprints of these stars, tracking back to 180 million years after the Big Bang.

                  Less than one million years after the Big Bang, the Universe consisted of atomic gas (chiefly hydrogen) and a form of matter that outweighs regular matter by more than five times but has yet to be seen directly. Measurements over decades have indicated that, oddly enough, this ‘dark’ matter interacts with itself and with regular matter only through the action of gravity. It was mainly the gravity of dark matter that amplified small, localized density perturbations in the Universe shortly after the Big Bang to generate the first large-scale structures. But it was the hydrogen within these perturbations that collapsed piecemeal to form stars, bringing about the cosmic dawn.

                  The observable thermal footprints of early stars derive from small variations in the ratio of the number of interstellar hydrogen atoms found in two particular energy states; a transition between these states causes a photon to be emitted or absorbed at a characteristic radio frequency. The ratio reflects the degree of excitation of the hydrogen, and can be expressed as a temperature, known as the atomic spin temperature (TS).

                  At early times, when the Universe was relatively small and mean gas density was high, collisions between atoms were frequent. TS was therefore the same as the kinetic temperature of the gas (TG), an indicator of the energy available to excite atoms through collisions. By the time stars began to form, the Universe had expanded. Both TG and mean gas densities had fallen, and collisions were infrequent, allowing TS to drift upward to the temperature of the radiation (TR) left over from the Big Bang (Fig. 1). TR also fell as the Universe expanded, but not as quickly as TG.

                  A long-standing theory that still awaits testing predicts that absorption of UV radiation from early stars by nearby clouds of hydrogen could have driven TS back down to TG, but not lower. In other words, the cosmic dawn would make the gas seem colder when observed at radio frequencies. This would create an absorption feature in the spectrum of the background radiation left over from the Big Bang.

                  Bowman et al. now report the possible detection of just such an absorption signal. The authors measured TS , averaged over much of the sky and over a contiguous range of radio frequencies; each frequency provides a window on a different time in the Universe’s past. The measurement is very difficult because it must be performed using an extremely well-calibrated VHF radio antenna and receiver, to enable the weak cosmological signal to be separated from much stronger celestial signals and from those within the electronics systems of the apparatus used.

                  The putative absorption signal extends over a wide frequency range, one end of which looks as far back as 180 million years ago, in good agreement with theoretical predictions6. Remarkably, however, the peak amplitude of the absorption is two to three times larger than predicted by the most optimistic models, and the absorption profile is flat-bottomed, rather than curvilinear and Gaussian-like, which is also at odds with models.

                  So how can the differences from the models be explained? In another paper in Nature, Barkana argues that models could achieve the reported signal amplitude and profile if non-gravitational interactions — like those that occur between charged particles — occur between dark matter and normal-matter particles, and if the dark-matter particles have relatively low masses and velocities that are less than the speed of light. The effects of variously hypothesized types of dark matter have been calculated previously, but only those in which dark matter and normal matter scatter each other increase the magnitude of the absorption signature. The idea that a detectable radio signal from the cosmic dawn can be connected to the particle properties of dark matter suggests a potentially revolutionary angle for exploring fundamental physics.

                  Bowman and colleagues’ claim to have detected the long-sought absorption signal is bolstered by myriad tests in which the authors altered their experimental hardware or data analysis, in a concerted effort to identify systematic errors that might be responsible for the measured signal. The tests included repeating the data acquisition and analysis using a duplicate antenna at a second, nearby location; orienting the antenna at different angles with respect to the compass; and changing the ways in which the antenna is isolated from the ground. Other tests focused on switching various facets of the data calibration on and off.

                  However, the most stringent test will be to compare the current results with those to come from independent experiments also aimed at detecting the cosmic-dawn signal. I hope that the unexpected amplitude and line shape of the reported absorption signal is indeed a hard-won breakthrough that reveals evidence of unexpected physics. But it is possible that systematic errors have escaped detection by the tests that were run. Two extensions to the reported tests include using circuitry that more precisely imitates the antenna than Bowman and colleagues’ circuitry when attached to the receiver during performance evaluation and calibration, and the cross-checking of performance models for the antenna (which are currently based on computer simulations of antenna electromagnetics) with field measurements made when narrowband or sinusoidal signals are broadcast near the antenna.

                  Bowman and co-workers’ report will be recognized as a milestone for this nascent experimental field: the first reputable claim of a much-anticipated detection. The follow-up will not be limited to ever finer interpretations of increasingly accurate one-dimensional spectra. Studies of the cosmic-dawn signal using interferometers (arrays of antennas) could describe the 3D structure of the Universe at that time and, by extrapolation, during the primordial ‘dark age’ when large-scale structure in the Universe first formed. One of Barkana’s particularly notable predictions is that, if non-gravitational interactions between normal and dark matter do exist, then the absorption signal detectable by interferometers could be stronger and more distinctive than had been predicted. It would encode the spatial fluctuations of matter density that occurred during the dark age, rather than just gas temperature, thus presenting new opportunities for tests of fundamental physics.

                   

                  #316655

                  Bjarne
                  Moderator
                    • Super Nova

                    An absorption profile centred at 78 megahertz in the sky-averaged spectrum

                    After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

                     

                    #316657

                    Bjarne
                    Moderator
                      • Super Nova
                      #316664

                      Bjarne
                      Moderator
                        • Super Nova

                        Jeg har nu fået læst artiklen. Observationerne er udført at 2 zenitteleskoper hver bestående af en enkelt dipolantenne placeret 150 m fra hinanden. De udførte målinger i frekvensområdet mellem 50 MHz og 100 MHz. Der er en kraftig synkrotronstråling fra Mælkevejen, men den kan sammen med andre bidrag modelleres med en langsomtvarierende funktion af frekvensen. Selve absorptionen fra 21-cm linien ses som et U-formet trug i den kosmiske mikrobølgebaggrundsstråling ved 78 MHz svarende til rødforskydningen z = 17. Forfatterne anvender en rent empirisk model for absorptionsprofilen, som ikke er baseret på en fysisk model. Signal/støj-forholdet for profilens amplitude er 37. Man har altså på ingen måde problemer med at detektere absorptionsprofilen. Man har med en anden dipol observeret i frekvensområdet 90 MHz og 200 Mhz uden at finde nogen absorption. Man har gjort sig store anstrengelser for at finde alternative forklaringer på absorptionsprofilen. Man ville under normale omstændigheder være tilfreds med de mange test af alle mulige støjkilder. Hvad er problemet ved disse målinger?

                        Det åbenlyse problem er bredden (19 MHz) af den U-formede absorption sammenlignet med bredden af det målte område (50 MHz). Den fysiske model for baggrunden bestemmes ud fra en båndbredde af kun ca. 30 MHz. Man kan derfor ikke undgå en covarians mellem parametrene for baggrundet og absorptionen. Dette har man taget højde for under databehandlingen, men der er sikkert plads til forbedringer. Et andet usikkerhedsmoment er kalibreringen af radiomodtager og dipolantenne. Artiklen har for øvrigt et diagram over radiomodtager og antenne, hvis nogen skulle være interesserede. Det er vigtigt, at andre grupper gentager eksperimentet med et uafhængigt udstyr. Hvis eksperimentet skal flyttes oven for ionosfæren, kan det kun foregå bag månen, da der ellers ville være direkte modtagelse af alle Jordens radiosendere.

                        Det helt store problem er standardmodellens forudsigelse af absorptionens dybde. Problemet er ikke så meget observationerne, men teorien. 21-cm linien fremkommer ved overgangen mellem 2 tilstande i hydrogens grundtilstand. Elektronen og protonen har samme spinretning i den øverste tilstand og modsat retning i den laveste tilstand. Energiforskellen svarer til frekvensen 1420 MHz (ca 21 cm). Forholdet mellem antallet af atomer i den øverste og den neterste tilstand angives ved den såkaldte spintemperatur TS. Hvis spintemperaturen er under baggrundsstrålingens temperatur TR vil der forekomme absorption. I modsat fald får man emission. Spintemperaturen vil altid være over hydrogengassens temperatur TG, så den maksimale styrke af absorptionen er under “normale” omstændigheder bestemt af forholdet TR/TG. Forklaring følger.

                        #316671

                        Bjarne
                        Moderator
                          • Super Nova

                          Jeg hentyder med betegnelsen “normale” omstændigheder til en periode i Universets ekspansion, hvor både fotoner og neutrale hydrogenatomer bevæger sig som frie partikler uden udveksling af energi med andre partikler. Det viser sig, at der findes en simpel formel for, hvordan en fri partikels impuls varierer med rødforskydningen z. En elektromagnetisk bølges bølgelængde λ varierer som alle andre afstande med Universets stalafaktor a(t), som er normeret til 1 for det nuværende tidspunkt t=t0, altså a(t0) = 1. En fotons bølgelængde varierer derfor som λ(t) = λ(t0)a(t). Rødforskydningen z for en foton til tiden t er givet ved:

                          1+z = λ(t0)/λ(t) = 1/a(t)

                          Man kan derfor anvende z i stedet for tiden t. Bølgelængden λ(z) som funktion af z er derfor givet ved

                          λ(z) = λ(0)/(1+z)

                          En fotons energi er E = hν = hc/λ. En fotons impuls er givet ved p = E/c = h/λ. Impulsen er altså omvendt proportional med bølgelængden, og der må derfor gælde, at impulsen p(z) som funktion af rødforskydningen z varierer på denne måde

                          p(z) = p(0)(1+z)

                          Man kan vise, at impulsen for en fri partikel med hvilemassen m varierer på nøjagtig samme måde under Universets ekspansion. Udtrykkene for partiklernes energi er imidlertid forskellige. En fotons energi er givet ved E = cp, hvor c er lyshastigheden. Et hydrogenatoms energi er givet ved

                          E = p2/(2m) = (1/2)mv2, hvor p = mv, og v er hastigheden.

                          E/kT er i begge tilfælde bevaret under Universets ekspansion, T er foton- eller hydrogengassens temperatur. k er Boltzmanns konstant. Ligningen definerer udviklingen af de to gassers temperaturer som funktion af rødforskydningen.

                          Om strålingstemperaturen TR gælder, at p(z)/TR(z) er konstant, så vi får

                          TR(z) = TR(0)(1+z)

                          Om gastemperaturen TG gælder, at p2(z)/TG(z) er konstant, så vi får

                          TG(z) = TG(0)(1+z)2

                          Formlen for den kosmiske mikrobølgebaggrundsstråling gælder altid, da energitætheden i baggrundsstrålingen er meget, meget større end den kinetiske energitæthed i hydrogengassen. Formlen for hydrogengassens temperatur gælder for rødforskydningsområder, hvor der ikke sker nævneværdig udveksling af energi med andre partikler.

                           

                          #316679

                          Bjarne
                          Moderator
                            • Super Nova

                            Protoner og elektroner begynder at finde sammen, at rekombinere som hydrogenatomer, ved rødforskydningen z = 1100. Den kosmiske mikrobølgebaggrundsstråling spredes på elektroner og holder hydrogengassen på samme temperatur som baggrundsstrålingen. Gassen fortsætter med af have samme temperatur som strålingen, indtil antallet af frie elektroner bliver bliver for lav til at opretholde en fælles temperatur. Herefter udvikler temperaturen for stråling TR og gas TG efter formlerne, som jeg udledte ovenfor. Forholdet mellem de to temperaturer udvikler sig derfor som

                            TG(z)/TR(z) = [TG(0)/TR(0)](1+z)

                            Numeriske beregninger viser, at temperaturenes veje skilles ved zeq = 150, hvor der gælder

                            1 = [TG(0)/TR(0)](1+zeq)

                            Jeg dividerer nu den nederste ligning med den øverste og får

                            TR(z)/TG(z) = (1+zeq)/(1+z)

                            Denne ligning gælder, indtil de første stjerner begynder at ionisere hydrogenatomerne ved z = 20, hvor absorptionstruget starter. Jeg kan nu beregne forholdet mellem strålingstemperaturen og gastemperaturen ved z = 20:

                            TR(20)/TG(20) = (1+150)/(1+20) = 7.2

                            Jeg har tidligere nævnt, at spintemperaturen TS altid er større end gastemperaturen, hvorfor der må gælde

                            TR/TS > 7

                            Dybden af den målte absorptionsprofil svarer imidlertid til TR/TS > 15 for den kosmologiske standardmodel. Dette er problemet i en nøddeskal.

                            #316683

                            Bjarne
                            Moderator
                              • Super Nova

                              Hvad er forklaringen på den bredere og kraftigere absorption ved 78 MHz end forventet ud fra ΛCDM modellen? Dette er uheldigvis et vanskeligt spørgsmål at besvare. Det er langt mindre oplagt end forklaringen på den “begejstrede” rapport fra sydpolen om detektion af gravitationsbølger. De fleste var klar over, at der var store problemer med korrektionen for baggrundspolarisation fra Mælkevejen. Man havde i realiteten bestemt en øvre grænse for kosmologiske gravitationsbølger. Men hvem ønsker at høre om en øvre grænse?

                              Jeg har ikke læst den tekniske beskrivelse af instrumentet. En mulig forklaring kan være kalibreringen af forstærker og antenne som funktion af frekvensen. Det er derfor vigtigt, at andre grupper foretager lignende observationer med uafhængige instrumenter og reduktionsmetoder. Jeg har nu set på kurverne for både amplituden og fasen for den målte antennereflektion. Jeg ser ikke noget, som bare ligner den U-formede absorptionsprofil. Mit bedste bud er, at absorptionsprofilen faktisk er reel.

                               

                              #316690

                              Bjarne
                              Moderator
                                • Super Nova

                                Jeg har fundet denne beskrivelse af det anvendte radioudstyr. Personerne bag udstyret er særdeles kompetente.

                                Experiment to Detect the Global EoR Signature (EDGES)

                                The Experiment to Detect the Global EoR Signature (EDGES) is a collaboration between ASU and MIT, with funding from the U.S. NSF and site support from the Australian CSIRO. The project aims to detect the global (all-sky) 21 cm signal through observations with individual dipole antennas. EDGES is located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia (-26.7148, 116.6044), which is the same site used by Australian SKA Precursor (ASKAP), the Murchison Widefield Array (MWA), and the future SKA Low-Frequency Aperture Array (LFAA). This location was chosen due to its radio-quiet conditions below 200 MHz. The experiment consists of two instruments: 1) a high-band instrument sensitive to 100-200 MHz, and 2) a low-band instrument sensitive to 50-100 MHz. The instruments are nearly identical except for their antennas and ground planes, which are scaled copies.

                                Siden viser billeder af udstyret.

                                 

                                #316723

                                Bjarne
                                Moderator
                                  • Super Nova

                                  Der er kommet en ny artikel om et forsøg på at at bestemme anisotropien (altså i forskellige retninger) for 21-cm signalet fra Universets reionisering. Dette viste sig (som forventet) meget, meget vanskeligere end blot at bestemme signalet fra en dipolantenne. Årsagen er, at radioemissionen fra Mælkevejen varierer meget med retningen. Dette var netop problemet med den fejlagtige bestemmelse af kosmologiske gravitationsbølger, som Holger omtalte. Problemet var her et kraftigt bidrag til polarisationen fra Mælkevejen. Man kan ikke i den nærmeste fremtid forvente en bekræftelse at 21-cm signalet fra interferometriske anisotropimålinger.

                                  EDGES gruppen har netop anvendt en simpel dipol, som er rettet mod zenit, for at minimere dette problem. Man har desuden valgt et øde sted med et minimum af radiointerferens fra mobiltelefoner og andre radiosendere. Den optimale følsomhed er opnået ved at eksperimentere med udstyrets udformning og kalibrering.

                                  Den teoretiske baggrund er, at man kender He- og hydrogenatomers vekselvirkning med stråling, samt at det mørke stof ikke vekselvirker med atomerne eller elektronerne. Man er derfor i stand til at beregne den maksimale dybde af 21-cm absorptionen ved begyndelsen af reioniseringen. Nu viser målingerne imidlertid, at absorptionen er bredere og dybere end forventet.

                                  Skal man være overrasket? Man har ikke været i stand til at detektere en eneste partikel som værende ansvarlig for det mørke stof. Man ved blot, at den største del af det mørke stof ikke vekselvirker med stråling ved temperaturer over 3000K. Resten er antagelser og formodninger.

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