| 1. |
- Beal, Jacob, et al.
(author)
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Robust estimation of bacterial cell count from optical density
- 2020
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In: Communications Biology. - : Springer Science and Business Media LLC. - 2399-3642. ; 3:1
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Journal article (peer-reviewed)abstract
- Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data.
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| 2. |
- Bernardo, Heliudson, et al.
(author)
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Modified Gravity Approaches to the Cosmological Constant Problem
- 2023
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In: Universe. - : MDPI. - 2218-1997. ; 9:2
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Research review (peer-reviewed)abstract
- The cosmological constant and its phenomenology remain among the greatest puzzles in theoretical physics. We review how modifications of Einstein's general relativity could alleviate the different problems associated with it that result from the interplay of classical gravity and quantum field theory. We introduce a modern and concise language to describe the problems associated with its phenomenology, and inspect no-go theorems and their loopholes to motivate the approaches discussed here. Constrained gravity approaches exploit minimal departures from general relativity; massive gravity introduces mass to the graviton; Horndeski theories lead to the breaking of translational invariance of the vacuum; and models with extra dimensions change the symmetries of the vacuum. We also review screening mechanisms that have to be present in some of these theories if they aim to recover the success of general relativity on small scales as well. Finally, we summarize the statuses of these models in their attempts to solve the different cosmological constant problems while being able to account for current astrophysical and cosmological observations.
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| 3. |
- Brandenburg, Axel, et al.
(author)
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Can we observe the QCD phase transition-generated gravitational waves through pulsar timing arrays?
- 2021
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In: Physical Review D. - : American Physical Society (APS). - 2470-0010 .- 2470-0029. ; 104:4
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Journal article (peer-reviewed)abstract
- We perform numerical simulations of gravitational waves (GWs) induced by hydrodynamic and hydromagnetic turbulent sources that might have been present at cosmological quantum chromodynamic (QCD) phase transitions. For turbulent energies of about 4% of the radiation energy density, the typical scale of such motions may have been a sizable fraction of the Hubble scale at that time. The resulting GWs are found to have an energy fraction of about 10(-9) of the critical energy density in the nHz range today and may already have been observed by the NANOGrav Collaboration. This is further made possible by our findings of shallower spectra proportional to the square root of the frequency for nonhelical hydromagnetic turbulence. This implies more power at low frequencies than for the steeper spectra previously anticipated. The behavior toward higher frequencies depends strongly on the nature of the turbulence. For vortical hydrodynamic and hydromagnetic turbulence, there is a sharp drop of spectral GW energy by up to five orders of magnitude in the presence of helicity, and somewhat less in the absence of helicity. For acoustic hydrodynamic turbulence, the sharp drop is replaced by a power law decay, albeit with a rather steep slope. Our study supports earlier findings of a quadratic scaling of the GW energy with the magnetic energy of the turbulence and inverse quadratic scaling with the peak frequency, which leads to larger GW energies under QCD conditions.
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| 4. |
- Brandenburg, Axel, et al.
(author)
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Relic Gravitational Waves from the Chiral Magnetic Effect
- 2021
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 911:2
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Journal article (peer-reviewed)abstract
- Relic gravitational waves (GWs) can be produced by primordial magnetic fields. However, not much is known about the resulting GW amplitudes and their dependence on the details of the generation mechanism. Here we treat magnetic field generation through the chiral magnetic effect (CME) as a generic mechanism and explore its dependence on the speed of generation (the product of magnetic diffusivity and characteristic wavenumber) and the speed characterizing the maximum magnetic field strength expected from the CME. When the latter exceeds the former (regime I), which is the regime applicable to the early universe, we obtain an inverse cascade with moderate GW energy that scales with the third power of the magnetic energy. When the generation speed exceeds the CME limit (regime II), the GW energy continues to increase without a corresponding increase of magnetic energy. In the early kinematic phase, the GW energy spectrum (per linear wavenumber interval) has opposite slopes in both regimes and is characterized by an inertial range spectrum in regime I and a white noise spectrum in regime II. The occurrence of these two slopes is shown to be a generic consequence of a nearly monochromatic exponential growth of the magnetic field. The resulting GW energy is found to be proportional to the fifth power of the limiting CME speed and the first power of the generation speed.
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| 5. |
- Brandenburg, Axel, et al.
(author)
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Simulations of Helical Inflationary Magnetogenesis and Gravitational Waves
- 2021
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 922:2
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Journal article (peer-reviewed)abstract
- Using numerical simulations of helical inflationary magnetogenesis in a low reheating temperature scenario, we show that the magnetic energy spectrum is strongly peaked at a particular wavenumber that depends on the reheating temperature. Gravitational waves (GWs) are produced at frequencies between 3 nHz and 50 mHz for reheating temperatures between 150 MeV and 3 x 10(5) GeV, respectively. At and below the peak frequency, the stress spectrum is always found to be that of white noise. This implies a linear increase of GW energy per logarithmic wavenumber interval, instead of a cubic one. Both in the helical and nonhelical cases, the GW spectrum is followed by a sharp drop for frequencies above the respective peak frequency. In this magnetogenesis scenario, the presence of a helical term extends the peak of the GW spectrum and therefore also the position of the aforementioned drop toward larger frequencies compared to the case without helicity. This might make a difference in it being detectable with space interferometers. The efficiency of GW production is found to be almost the same as in the nonhelical case, and independent of the reheating temperature, provided the electromagnetic energy at the end of reheating is fixed to be a certain fraction of the radiation energy density. Also, contrary to the case without helicity, the electric energy is now less than the magnetic energy during reheating. The fractional circular polarization is found to be nearly 100% in a certain range below the peak frequency range.
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| 6. |
- He, Yutong, 1996-
(author)
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Cosmological gravitational waves and their interaction with large-scale magnetic fields
- 2024
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Doctoral thesis (other academic/artistic)abstract
- The thesis explores the interactions between cosmological gravitational waves (GWs) and large-scale magnetic fields. GWs are radiation produced by spacetime variations of the stress-energy tensor. Due to the weak coupling between gravity and the matter sector, GWs are a unique messenger from the early Universe before the Cosmic Microwave Background (CMB). Magnetic fields are observed across the Universe from the scales of planets and stars to galaxies and clusters, as well as the voids beyond the clusters. The present-day large-scale magnetic fields are believed to have evolved from primordial seed fields via magnetogenesis mechanisms active during the cosmic inflation and reheating epochs or the cosmological phase transitions occurring at the electroweak (EW) or quantum chromodynamic (QCD) scales in the early radiation-dominated (RD) era. The production of stochastic GW backgrounds (SGWBs) can be expected from the primordial electromagnetic (EM) fields or magnetohydrodynamic (MHD) turbulence around the time of EW and QCD phase transitions. The SGWBs then propagate through the pre-CMB Universe until the present day, carrying with them essential imprints of the corresponding sources as well as the underlying gravity theory at the early times. Since MHD turbulence is ubiquitously expected in the RD era, their induced SGWB spectrum is extensively studied.In one aspect of the thesis, we demonstrate that the MHD-GW system exhibits features of modified gravity (MG) in terms of the spectral slopes and amplitudes of the relic SGWB. We compute the spectra of GWs produced by MHD turbulence at the EW and QCD phase transitions, assuming massive gravity and scalar-tensor theories as two MG examples. Then we comparatively analyze these modified GW spectra with their counterparts in general relativity, and determine their qualitative and quantitative differences due to three effective MG parameters – graviton mass, GW friction, and GW speed. These spectral features are compared against the existing pulsar timing array (PTA) measurement in the nHz band as well as the expected GW detection sensitivities of upcoming instruments such as the Laser Interferometer Space Antenna (LISA) in the mHz band and Square Kilometer Array (SKA) as a PTA. The framework is general and can be applied to non-MHD sourced GW spectra. However, fixing MHD sources yields concrete constraints on the effective MG parameters.The other aspect of the thesis concerns the interaction between SGWBs from the early Universe and large-scale magnetic fields in the post-CMB Universe, which would convert a fraction of the gravitons in the SGWBs into photons of the same frequency via the inverse Gertsenshtein effect. The graviton-induced photons could then be a source of spectral distortions of the blackbody CMB.This, in turn, would allow us to constrain the GW amplitudes in the MHz-THz frequency regime. The high-frequency GWs (HFGWs) correspond to new physics in the early Universe such as beyond-Standard-Model mechanisms or sub-stellar mass primordial black holes. We place constraints on the HFGW energy densities by exploiting the existing direct observations of the radio sky, measurements of the 21-cm signal upper limits, the kinematic Sunyaev-Zeldovich observations, and assuming that graviton-induced photons saturate all of the reported radio excess over the CMB. We also forecast the potential of SKA and proposed future CMB surveys as novel HFGW detectors, and show that they will significantly tighten the current constraints and bring us a step closer to detecting HFGWs.
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| 7. |
- He, Yutong
(author)
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Gravitational waves from primordial magnetic sources
- 2022
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Licentiate thesis (other academic/artistic)abstract
- This licentiate thesis gives an overview of modified gravity and primordial magnetic fields,bridged by gravitational waves (GWs) as a phenomenological probe of both sectors.First we derive the GW equation in a homogeneous and isotropic background from general relativity.This is followed by a brief review of modified gravity -- its motivations and some common approaches.Within the vast theory space of modified gravity, however, only a handful of parameters enter at the level of the GW equation, in a relatively model-independent manner.We then discuss a range of possible mechanisms giving rise to GWs in the early universe -- from inflation to the early radiation era -- and note, in particular, that magnetically-driven turbulence could be present regardless of the orders of the phase transitions.Therefore, the anisotropic stress due to primordial magnetic fields could be an ubiquitous category of production mechanisms for GWs,whose present-day relic spectrum could then be used to constrain the parameters of modified gravity, in addition to inferring the nature of the primordial magnetic sources themselves.
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| 8. |
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| 9. |
- He, Yutong, et al.
(author)
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Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves
- 2024
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 2024:5
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Journal article (peer-reviewed)abstract
- We apply the inverse Gertsenshtein effect, i.e., the graviton-photon conversion in the presence of a magnetic field, to constrain high-frequency gravitational waves (HFGWs). Using existing astrophysical measurements, we compute upper limits on the GW energy densities ΩGW at 16 different frequency bands. Given the observed magnetisation of galaxy clusters with field strength B ∼ μG correlated on O(10) kpc scales, we estimate HFGW constraints in the O(102) GHz regime to be ΩGW ≲ 1016 with the temperature measurements of the Atacama Cosmology Telescope (ACT). Similarly, we conservatively obtain ΩGW ≲ 1013 (1011) in the O(102) MHz (O(10) GHz) regime by assuming uniform magnetic field with strength B ∼ 0.1 nG and saturating the excess signal over the Cosmic Microwave Background (CMB) reported by radio telescopes such as the Experiment to Detect the Global EoR Signature (EDGES), LOw Frequency ARray (LOFAR), and Murchison Widefield Array (MWA), and the balloon-borne second generation Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE2) with graviton-induced photons. The upcoming Square Kilometer Array (SKA) can tighten these constraints by roughly 10 orders of magnitude, which will be a step closer to reaching the critical value of ΩGW = 1 or the Big Bang Nucleosynthesis (BBN) bound of ΩGW ≃ 1.2 × 10-6. We point to future improvement of the SKA forecast and estimate that proposed CMB measurement at the level of O(100-2) nK, such as Primordial Inflation Explorer (PIXIE) and Voyage 2050, are needed to viably detect stochastic backgrounds of HFGWs.
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| 10. |
- He, Yutong, et al.
(author)
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Modified propagation of gravitational waves from the early radiation era
- 2023
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :6
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Journal article (peer-reviewed)abstract
- We study the propagation of cosmological gravitational wave (GW) backgrounds from the early radiation era until the present day in modified theories of gravity. Comparing to general relativity (GR), we study the effects that modified gravity parameters, such as the GW friction & alpha;M and the tensor speed excess & alpha;T, have on the present-day GW spectrum. We use both the WKB estimate, which provides an analytical description but fails at superhorizon scales, and numerical simulations that allow us to go beyond the WKB approximation. We show that a constant & alpha;T makes relatively insignificant changes to the GR solution, especially taking into account the constraints on its value from GW observations by the LIGO-Virgo collaboration, while & alpha;M can introduce modifications to the spectral slopes of the GW energy spectrum in the low-frequency regime depending on the considered time evolution of & alpha;M. The latter effect is additional to the damping or growth occurring equally at all scales that can be predicted by the WKB approximation. In light of the recent observations by pulsar timing array (PTA) collaborations, and the potential observations by future detectors such as SKA, LISA, DECIGO, BBO, or ET, we show that, in most of the cases, constraints cannot be placed on the effects of & alpha;M and the initial GW energy density E*GW separately, but only on the combined effects of the two, unless the signal is observed at different frequency ranges. In particular, we provide some constraints on the combined effects from the reported PTA observations.
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