Tag Archives: universe

Scientists Explore Differences in Extragalactic Jets Emerging From Black Holes

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A team of international astrophysicists have uncovered new insights into the mystery behind the differences in the appearances of extragalactic jets emerging from the environments of supermassive blackholes. They showed that the plasma composition can affect the appearances of these jets. This may help to unravel the mystery of matter content of relativistic jets.

At the centers of many distant galaxies reside supermassive black holes with masses millions to billions of times that of our Sun. These black holes don’t just eat everything, but can also act like powerful engines, launching narrow beams of plasma and energy known as “jets” that shoot into space at nearly the speed of light. These extragalactic jets can travel for thousands of light-years and emit radiation ranging from low-energy radio waves to high-energy gamma rays.

For a long time, astronomers have been wondering about a noticeable difference in radio images of extragalactic jets, first identified by Fanaroff & Riley in 1974. They broadly classified radio jets into two main categories: FR I & FR II. The FR I jets are “core-brightened,” meaning they are brightest near the core and gradually fade into diffuse structures as they move outward. The FR II jets, on the other hand, are “edge-brightened,” meaning they are fainter near the core but stay tightly focused over long distances until they hit the surrounding gas, creating giant “hot spots” at their tips.

Scientists have for long continued to debate whether this difference is due to the black hole itself, the environment around it, or the intrinsic properties of the jet, such as its speed, temperature, and magnetic strength, etc.

A new research published in The Astrophysical Journal by Mr. Priyesh Kumar Tripathi, Dr. Indranil Chattopadhyay, and Mr. Sanjit Debnath from Aryabhatta Research Institute of Observational Sciences (ARIES), Dr. Raj Kishore Joshi from the Nicolaus Copernicus Astronomical Center, Poland, Dr. Ritaban Chatterjee from Presidency University, Kolkata, and Dr. M. Saleem Khan from MJPRU Barelly, used advanced computer simulations to reveal that the secret to these differences may be due to the jet’s composition and the environment it travels through. The research team performed large 3D magnetohydrodynamic (MHD) simulations of these jets at kiloparsec scales using a numerical simulation code developed by the Numerical and Theoretical Astrophysics Group at ARIES. Notably, this code incorporates a relativistic equation of state, which can accurately handle a very large range of temperatures encountered at different regions of the jet.

The team discovered that a phenomenon called the “kink instability” is a major player in shaping these powerful, narrow jets, causing wiggles (small bend). In space, if this wiggle grows faster than the jet can flow forward, the jet beam disrupts, spreading its energy into a faint, diffuse cloud – the classic look of an FR I jet. Astrophysical jets aren’t made of ordinary matter. Instead, they are composed of plasma, a soup of charged particles including electrons, positrons (the antimatter twin of electrons), and sometimes heavier particles like protons. One of the study’s most significant findings is that the composition of jet plasma can determine its fate.

Jets can be made of mostly electrons and protons (Hadronic plasma), a mixture that includes positrons (the antimatter twin of the electron– Leptonic/Mixed plasma).

Fig: 3D Volume rendering of the jet tracer for electron-proton and mixed plasma jet

The simulations showed that jets rich in positrons (lepton-rich) are relatively hotter, causing them to expand and slow down. They often can’t stay straight and get twisted by the kink instability. As a result, they form a diffuse, FR I–like structure, where the jet gradually fades instead of ending in a bright hotspot. In contrast, jets composed primarily of electrons and protons were more likely to transition between morphologies, thereby changing their identity. This suggests that what we see through our telescopes might just be a snapshot of a long, evolving cosmic process.

 

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No Dark Matter, astronomers find the long missing Universe’s ordinary matter

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Astronomers have detected much of the Universe’s ordinary matter, which had long been missing from accounts of its total mass. Not ‘dark matter’ — the mysterious, invisible stuff that makes up the majority of the Universe’s contents. This is normal matter, but it’s spread so sparsely across intergalactic space that more than three-quarters of it is almost undetectable.

Using an array of 36 radio telescopes in remote Western Australia, researchers analysed the light from 6 fast radio bursts (FRBs), unusually energetic events that last just milliseconds and originate in other galaxies. The spectrum was sensitive enough to reveal the exceedingly thin matter that the FRBs met in their travels. “The missing matter was equivalent to only one or two atoms in a room the size of an average office,” says radio astronomer Jean-Pierre Macquart.

More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy clusters. Censuses of the nearby Universe have used absorption line spectroscopy to observe the ‘invisible’ baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe’s volume and probably most of its mass.

Universe’s invisible baryons

In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas in denser regions near galaxies. Other techniques to observe these invisible baryons also have limitations — Sunyaev–Zel’dovich analyses can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters.

The scientists said a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon.

“We augment the sample of reported arcsecond-localized fast radio bursts with four new localizations in host galaxies that have measured redshifts of 0.291, 0.118, 0.378 and 0.522. This completes a sample sufficiently large to account for dispersion variations along the lines of sight and in the host-galaxy environments, and we derive a cosmic baryon density of Ωb=0.051+0.021−0.025h−170 (95 per cent confidence; h70 = H0/(70 km s−1 Mpc−1) and H0 is Hubble’s constant,” wrote scientists in their paper published in Nature.

This independent measurement is consistent with values derived from the cosmic microwave background and from Big Bang nucleosynthesis, they wrote in their abstract.

Stephen Hawking’s final theory on Big Bang published, What it says?

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Professor Stephen Hawking’s final theory on the origin of the universe, predicting the universe is finite and far simpler than many current theories on it, has been published on Wednesday, April 2, 2018 in the Journal of High Energy Physics.

The theory, worked in collaboration with Professor Thomas Hertog from KU Leuven, was submitted for publication before Hawking’s death earlier this year.

Modern theories of the big bang predict that our local universe came into existence due to inflation within a tiny fraction of a second after the big bang itself, and the universe expanded at an exponential rate. “The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean,” said Hawking in an interview last year.

In their new paper, Hawking and Hertog say this account of eternal inflation is wrong. “It assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats the quantum effects as small fluctuations around this,” said Hertog. “However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down in eternal inflation.”

On his part, Hawking said, “We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure.”

The theory of eternal inflation that Hawking and Hertog put forward is based on string theory concept of holography, which postulates that the universe is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.

Hawking’s earlier ‘no boundary theory’ predicted that if you go back in time to the beginning of the universe, the universe shrinks and closes off like a sphere, but this new theory represents a different interpretation. “Now we’re saying that there is a boundary in our past,” said Hertog.

 

Hertog now plans to study the implications of the new theory on smaller scales within the reach of our space telescopes. He believes that primordial gravitational waves – ripples in space time – generated at the exit from eternal inflation constitute the most promising “smoking gun” to test the model.

The expansion of our universe since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors, which can be heard by the planned European space-based gravitational wave observatory, LISA.