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C. Sivaram

Publications and source records attributed to C. Sivaram.

At least 19 recordsLinked to original sources
arXiv2024

Making Habitable Worlds: Planets Versus Megastructures

In 2016, a star KIC 8462852 caught the world's attention due to a paper by citizen scientists who noticed its seemingly unexplainable brightness variations. The forward theory was offered - KIC 8462852 is surrounded by a Dyson sphere, a megastructure made by an alien civilization to collect all energy output from their star. Finally, in 2018, its light curve showed chromaticity more characteristic of the dust (from comets or asteroids) rather than of something made from solid material, but the world was woken up to the idea of megastructures. But, in Dyson's time, only Solar System planets were known; it took more than 20 years to realize that nature has no problem making planets and does it with a flair -- the total number of planets in the Galaxy is estimated to be in billions. With such abundance of planets, there would be no need to destroy the entire planetary system to make one sphere. Instead, a civilization can expand to a system that has planet(s) in the habitable zone (HZ), or a planet can be moved into it. Alternatively, a free-floating planet (FFP) can be captured and moved into the HZ. These shifts can be performed at a constant low-thrust acceleration using high power directional lasers, resulting in a gradual spiral transfer from one orbit to another. We propose here to search for ETI by looking for high-power laser technosignatures and consider merits of such signatures. We suggest to specifically pay attention to the multiple planetary systems that have Strange Exoplanetary Architectures (SEA) - unusual planetary arrangements that cannot be explained by current planetary formation theories, because these could be the result of ETI moving planets intentionally to suit their needs.

physics.pop-ph↗
arXiv2022

From maximum force to the field equations of general relativity -- and implications

There are at least two ways to deduce Einstein's field equations from the principle of maximum force $c^4/4G$ or from the equivalent principle of maximum power $c^5/4G$. Tests in gravitational wave astronomy, cosmology, and numerical gravitation confirm the two principles. Apparent paradoxes about the limits can all be resolved. Several related bounds arise. The limits illuminate the beauty, consistency and simplicity of general relativity from an unusual perspective.

gr-qc↗
arXiv2016

Classical space-time as Rydberg states of underlying quantum geometries

Classical macroscopic space-time is pictured in terms of Rydberg states of an underlying discritzed `atomic' quantum geometry at Planck scales. While quantum geometry on such scales involves several very short lived transitions changing curvature and topologies, the Rydberg states have very long lifetimes, going as a high power of the quantum number n. This means space-time on macroscopic scales are almost infinitely stable. The large degeneracy in the Rydberg levels, with high n, can also account for a large black hole entropy, as well as long lifetime of massive black holes to quantum decays. We have a possible promising paradigm to link quantum geometry at Planck scales, to classical space-time.

gr-qc↗
arXiv2013

Some consequences of a Universal Tension arising from Dark Energy for structures from Atomic Nuclei to Galaxy Clusters

In recent work, a new cosmological paradigm implied a mass-radius relation, suggesting a universal tension related to the background dark energy (cosmological constant), leading to an energy per unit area that holds for structures from atomic nuclei to clusters of galaxies. Here we explore some of the consequences that arise from such a universal tension.

physics.gen-ph↗
arXiv2013

Is there Lower Limit to Velocity or Velocity Change?

Here we explore the possibility of a lower limit to velocity or velocity change which is 20 orders of magnitude smaller than the speed of light and explore the various observable signatures including those in cosmic rays and gamma ray bursts.

physics.gen-ph↗
arXiv2013

Holography, Dark Energy and Entropy of Large Cosmic Structures

As is well known, black hole entropy is proportional to the area of the horizon suggesting a holographic principle wherein all degrees of freedom contributing to the entropy reside on the surface. In this note, we point out that large scale dark energy (such as a cosmological constant) constraining cosmic structures can imply a similar situation for the entropy of a hierarchy of such objects.

physics.gen-ph↗
arXiv2012

Extended Equivalence Principle: Implications for Gravity, Geometry and Thermodynamics

The equivalence principle was formulated by Einstein in an attempt to extend the concept of inertial frames to accelerated frames, thereby bringing in gravity. In recent decades, it has been realised that gravity is linked not only with geometry of space-time but also with thermodynamics especially in connection with black hole horizons, vacuum fluctuations, dark energy, etc. In this work we look at how the equivalence principle manifests itself in these different situations where we have strong gravitational fields. In recent years the generalised uncertainty principle has been invoked to connect gravity and curvature with quantum physics and now we may also need an extended equivalence principle to connect quantum theory with gravity.

physics.gen-ph↗
arXiv2012

Some Unique Constants Associated with Extremal Black Holes

In recent papers we had developed a unified picture of black hole entropy and curvature which was shown to lead to Hawking radiation. It was shown that for any black hole mass, holography implies a phase space of just one quantum associated with the interior of the black hole. Here we study extremal rotating and charged black holes and obtain unique values for ratios of angular momentum to entropy, charge to entropy, etc. It turns out that these ratios can be expressed in terms of fundamental constants in nature, having analogies with other physical systems, like in condensed matter physics.

physics.gen-ph↗
arXiv2011

Some more Exotic Dark Matter Candidates: GUT Balls, Fermi Balls...

The nature of dark matter (DM), which is supposed to constitute about one-fourth of the universe, is still a mystery. There is evidence that much of the DM may be made up of as yet undiscovered particles with several experiments all over the world trying to detect these. In this article we introduce some new candidates which are exotic in nature but still are consistent with known physics. We look at DM objects that can be formed by the balance of gravity with the four-fermion force, nuclear tension, etc. We see that their radii are much larger than their corresponding Schwarzschild radius; hence they are distinct from Hawking primordial black holes. We have determined their mass and the required number densities to account for the DM in the galaxy and possible ways of detecting them.

physics.gen-ph↗
arXiv2011

Hydrodynamics, horizons, holography and black hole entropy

The usual discussions about black hole dynamics involve analogies with laws of thermodynamics especially in connection with black hole entropy and the associated holographic principle. We explore complementary aspects involving hydrodynamics of the horizon geometry through the membrane paradigm. New conceptual connections complementing usual thermodynamic arguments suggest deep links between diverse topics like black hole decay, quantum circulation and viscosity. Intriguing connections between turbulence cascades, quantum diffusion via quantum paths following Fokker- Planck equation and Hawking decay also result from this combination of thermodynamic and hydrodynamic analogies to black hole dynamics.

physics.gen-ph↗
arXiv2011

Dieterici gas as a Unified Model for Dark Matter and Dark Energy

The dominance of dark energy in the universe has necessitated the introduction of a repulsive gravity source to make q0 negative. The models for dark energy range from a simple lambda-term to quintessence, Chaplygin gas, etc. We look at the possibility of how change of behaviour of missing energy density, from DM to DE, may be determined by the change in the equation of state of a background fluid instead of a form of potential. The question of cosmic acceleration can be discussed within the framework of theories which do not necessarily include scalar fields.

physics.gen-ph↗
arXiv2010

A Critique of Drexler Dark Matter

Drexler dark matter is an alternate approach to dark matter that assumes that highly relativistic protons trapped in the halo of the galaxies could account for the missing mass. We look at various energetics involved in such a scenario such as the energy required to produce such particles and the corresponding lifetimes. Also we look at the energy losses from synchrotron and inverse Compton scattering and their signatures. The Coulomb repulsive instability due to the excess charge around the galaxies is also calculated. The above results lead us to conclude that such a model for DM is unfeasible.

physics.gen-ph↗
arXiv2010

Double Detonation of Sub-Chandrasekhar White Dwarfs and Subluminous Type Ia Supernovae

Type Ia supernovae are thought to result from thermonuclear explosions of carbon-oxygen white dwarf stars. This model generally explains the observed properties with certain exceptions, like sub-luminous supernovae. Here we discuss the possibility of sub-Chandrasekhar WDs detonating due to the build up of a layer of helium on the C-O WD by accreting from a helium rich companion star to explain observed deviations such as subluminous type Ia. We also detail some of the energetics involved that will make such scenarios plausible.

astro-ph.SR↗
arXiv2010

Gamma Ray Burst triggering Supernova Explosion (and other effects on neighbouring stars)

The initial burst of a gamma ray burst (GRB) is usually followed by a longer-lived afterglow emitted at longer wavelengths. The evidence for a physical connection between GRBs and core collapse supernovae (SN) has increased since the discovery of GRB afterglows. So far SN signatures have been found in only a few GRBs. Here we propose the possibility of a GRB triggering the collapse of a WR or RG star in a binary system producing a SN, and typical signatures. We also look at the effects of GRBs on MS and WD stars in the neighbourhood. The possibility of GRBs retarding star formation in an interstellar cloud is also discussed.

physics.gen-ph↗
arXiv2010

Possible Upper limits on Lorentz Factors in High Energy Astrophysical Processes

Gamma ray bursts (GRBs) are the most luminous physical phenomena in the universe. The relativistic effect on the blast wave associated with the GRB introduces the gamma factor. Here we put an upper limit on the gamma factor via constraints on maximal power allowed by general relativity and hence set upper limits on other observable quantities such as deceleration distance. Also upper limits are set on the high energy particle radiation due to constraints set by cosmic microwave background radiation.

physics.gen-ph↗
arXiv2010

Can Population III Stars at High Redshifts produce GRB's?

Gamma ray bursts are the most luminous physical phenomena in the universe, consisting of flashes of gamma rays that last from seconds to hours. There have been attempts to observe gamma ray bursts, for example, from population III stars of about 500 solar mass at high redshifts. Here we argue that collapse of such high mass stars does not lead to gamma ray burst as their core collapse temperatures are not sufficient to produce gamma rays, leading to GRBs.

physics.gen-ph↗
arXiv2010

Big Bang Nucleosynthesis and Primordial Black Holes

There are ongoing efforts in detecting Hawking radiation from primordial black holes (PBH) formed during the early universe. Here we put an upper limit on the PBH number density that could have been formed prior to the big bang nucleosynthesis era, based on the constraint that the PBH evaporation energy consisting of high energy radiation not affect the observed abundances' of elements, by disintegrating the nuclei.

physics.gen-ph↗