There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

/pdf/the-observation-of-gravitational-waves-from-a-binary-black-58srpupgg9.pdf

The Observation of Gravitational Waves from a Binary Black Hole Merger

Unified cosmic history in modified gravity: from F(R) theory to Lorentz non-invariant models

Quantum Non-linear Sigma-Models

We find the natural embedding of the (R+R^2)-inflationary model into the recently constructed N=1 F(\cal R)-supergravity. It gives a simple and viable realization of chaotic inflation in supergravity. The only requirement for a slow-roll inflation is the existence of the (\cal R)^3-term with an anomalously large coefficient in Taylor expansion of the F(\cal R) function, where \cal R is the covariantly-chiral scalar supercurvature superfield.

Embedding (R+R^2)-Inflation into Supergravity

We study a specific model of anisotropic strange stars in the modified |$f\left(R,\mathcal {T}\right)$|-type gravity by deriving solutions to the modified Einstein field equations representing a spherically symmetric anisotropic stellar object. We take a standard assumption that |$f(R,\mathcal {T})=R+2\chi \mathcal {T}$|⁠, where R is Ricci scalar, |$\mathcal {T}$| is the trace of the energy–momentum tensor of matter, and χ is a coupling constant. To obtain our solution to the modified Einstein equations, we successfully apply the ‘embedding class one’ techniques. We also consider the case when the strange quark matter (SQM) distribution is governed by the simplified MIT bag model equation of state given by |$p_r=\frac{1}{3}\left(\rho -4B\right)$|⁠, where B is bag constant. We calculate the radius of the strange star candidates by directly solving the modified TOV equation with the observed values of the mass and some parametric values of B and χ. The physical acceptability of our solutions is verified by performing several physical tests. Interestingly, besides the SQM, another type of matter distribution originates due to the effect of coupling between the matter and curvature terms in the |$f\left(R,\mathcal {T}\right)$| gravity theory. Our study shows that with decreasing the value of χ, the stellar systems under investigations become gradually massive and larger in size, turning them into less dense compact objects. It also reveals that for χ < 0 the |$f\left(R,\mathcal {T}\right)$| gravity emerges as a suitable theory for explaining the observed massive stellar objects like massive pulsars, super-Chandrasekhar stars, and magnetars, etc., which remain obscure in the standard framework of General Relativity.

Exploring physical features of anisotropic strange stars beyond standard maximum mass limit in $f\left(R,\mathcal {T}\right)$ gravity

Inflationary slow-roll dynamics in Einstein gravity with a nonminimal scalar-curvature coupling can be equivalent to that in the certain f(R) gravity theory. We review the correspondence and extend it to N=1 supergravity. The nonminimal coupling in supergravity is rewritten in terms of the standard (`minimal') N=1 matter-coupled supergravity by using curved superspace. The established equivalence between two different inflationary theories means the same inflaton scalar potential, and does not imply the same post-inflationary dynamics and reheating.

https://iopscience.iop.org/article/10.1088/1475-7516/2012/08/022/pdf

Inflation and nonminimal scalar-curvature coupling in gravity and supergravity

Inflationary slow-roll dynamics in Einstein gravity with a non-minimal scalar-curvature coupling can be equivalent to that in the certain f(R) gravity theory. We review the correspondence and extend it to N=1 supergravity. The non-minimal coupling in supergravity is rewritten in terms of the standard (`minimal') N=1 matter-coupled supergravity by using curved superspace. The established equivalence between two different inflationary theories means the same inflaton scalar potential, and does not imply the same post-inflationary dynamics and reheating.

Sergei V. Ketov

Papers

Quantum Non-linear Sigma-Models

Embedding (R+R^2)-Inflation into Supergravity

Exploring physical features of anisotropic strange stars beyond standard maximum mass limit in $f\left(R,\mathcal {T}\right)$ gravity

Inflation and nonminimal scalar-curvature coupling in gravity and supergravity

Inflation and non-minimal scalar-curvature coupling in gravity and supergravity