Now We Know That Dark Matter Isn’t Primordial Black Holes

For over fifty years, scientists have theorized that roughly 85% of matter in the Universe’s is made up of a mysterious, invisible mass. Since then, multiple observation campaigns have indirectly witnessed the effects that this “Dark Matter” has on the Universe. Unfortunately, all attempts to detect it so far have failed, leading scientists to propose some very interesting theories about its nature.

One such theory was offered by the late and great Stephen Hawking, who proposed that the majority of dark matter may actually be primordial black holes (PBH) smaller than a tenth of a millimeter in diameter. But after putting this theory through its most rigorous test to date, an international team of scientists led from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) has confirmed that it is not.

The team was led by Hiroko Niikura, a PhD candidate student with the Kavli IPMU, and included researchers from Japan, India and the US. As they indicate in their paper, which recently appeared in the journal Nature Astronomy, the lack of results in dark matter research led them to consider Hawking’s theory, which he first suggested in 1974.

This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. Credit: NASA, ESA, L. Calcada

This theory posits that the majority of dark matter is made up of primordial black holes (PBH) formed shortly after the Big Bang. As Prof. Masahiro Takada, the principal investigator of the IMPU and a co-author on the paper, told Universe Today via email:

“What Prof. Hawking predicted was that, if there were little patches of overdensity in the early Universe, such patches could create black holes… Once black holes are formed, they would behave like dark matter (because it is invisible and interacts with other particles only via gravity).”

This theory is attractive because it does not rely on the existence of any exotic (but as of yet, undiscovered) particles. What’s more, shortly after Hawking proposed this idea, astrophysicists discovered that cosmic inflation could generate patches of overdensity in the early Universe due to quantum fluctuations, which could have resulted in black holes.

The team tested this theory by using the Subaru Telescope at the Mauna Kea Observatory in Hawaii to observe the neighboring Andromeda Galaxy, which is located about 2.54 million light years away. Unlike most galaxies in our cosmic neighborhood, Andromeda is one of only 100 or so that is approaching our galaxy – at a rate of 110 km per second (68 mi per second) – and is destined to collide with it.

Andromeda Galaxy. Credit: Wikipedia Commons/Adam Evans

These and other factors contributed to it being the best candidates to test Hawking’s theory, explained Prof. Takada:

“The Andromeda galaxy is the largest, nearby galaxy containing many stars inside. For example, Andromeda is much bigger than the Magellan clouds that are dwarf galaxies. Hence, IF we can observe stars in Andromeda at one time, we could find microlensing events of a star, the flicker of its brightness, due to a foreground black holes that are passing in front of the star on the sky.”

If Hawking’s theory were in fact correct, the space between Andromeda and our galaxy would be filled with PBHs. This would result in a gravitational lensing effect, where the gravitational force of all these tiny black holes would cause the light rays coming from Andromeda’s stars to bend and become magnified.

This effect, which was first predicted by Einstein and his Theory of General Relativity in 1915, has been used many times by astronomers to view distant objects by taking advantage of the presence of massive objects in between them and Earth. However, the opportunities for such events are rare, requiring a fortuitous alignment between the observer, the distant object and the intervening one.

To maximize their chances of capturing an event, the researchers team used the Subaru Telescope’s Hyper Suprime-Cam digital camera, which is able to capture whole images of the Andromeda galaxy in a single shot. They also took multiple images of the galaxy to make sure that they caught any brief flickers coming from Andromeda’s stars.

Artist’s conception shows two merging black holes similar to those detected by LIGO on January 4th, 2017. Credit: LIGO/Caltech

These flickers would indicate that a primordial black hole was passing in front of them, thus distorting and magnifying their light. As Prof. Takada explained:

“If dark matter is PBH rather than elementary particles such as WIMPs, it could pass in front of a star in Andromeda galaxy and cause a microlensing event, the flicker of its brightness changing with observation time. This timescale in the change of star brightness depends on mass and velocity of PBH. If PBH is dark matter, we have a good understanding of its velocity: it should move with ~200km/s in interstellar space as the rotation curve of our Milky Way or Andromeda galaxy shows.”

All told, the team took 190 consecutive images of Andromeda over the course of seven hours and examined them closely for indications of a possible event. Given the expected mass of primordial black holes, at least 1000 events were anticipated. However, the team found evidence of only one event, which would indicate that primordial black holes would constitute less than 0.1% of dark matter mass.

That being said, this one possible event (which lasted for about an hour) was a significant discovery, since it is precisely what astronomers would expect for a light-mass PBH. As Takada indicated, this could be indirect evidence of a PBH caused by cosmic inflation. At the same time, it could be evidence of stellar variability (i.e. a stellar flare), so more observations are necessary before anything can be said definitively.

Illustris simulation, showing the distribution of dark matter in 350 million by 300,000 light years. Galaxies are shown as high-density white dots (left) and as normal, baryonic matter (right). Credit: Markus Haider/Illustris

Looking ahead, the team is planning on conducting further observations of the Andromeda galaxy to confirm their analysis. They also hope to investigate a new theory that posits how binary black holes – which have become detectable by LIGO thanks to the gravitational waves events they create – might in fact be primordial black holes.

“In brief, our results can’t entirely exclude the PBH-dark matter scenario, so dark matter could be an unknown elementary particle such as Weakly Interacting Massive Particle (WIMP),” concludes Prof. Takada. “In this case, we hope that underground experiments or accelerator experiments such as LHC will find such dark matter particles.”

In the meantime, the search for elusive dark matter continues! And much like the first-ever detection of gravitational waves, this discovery will trigger a revolution in the field of astronomy. And as Takada said, “it would be a Novel Prize discovery!”

Further Reading: Kavli IPMU, Nature Astronomy

Fuente: https://www.universetoday.com/141923/now-we-know-that-dark-matter-isnt-primordial-black-holes/

Pulsar Seen Speeding Away From the Supernova That Created it

When a star exhausts its nuclear fuel towards the end of its lifespan, it undergoes gravitational collapse and sheds its outer layers. This results in a magnificent explosion known as a supernova, which can lead to the creation of a black hole, a pulsar or a white dwarf. And despite decades of observation and research, there is still much scientists don’t know about this phenomena.

Luckily, ongoing observations and improved instruments are leading to all kinds of discoveries that offer chances for new insights. For instance, a team of astronomers with the National Radio Astronomy Observatory (NRAO) and NASA recently observed a “cannonball” pulsar speeding away from the supernova that is believed to have created it. This find is already providing insights into how pulsars can pick up speed from a supernova.

The pulsar, which is designated PSR J0002+6216 (J0002), is located about 6,500 light-years from Earth. It was originally discovered in 2017 by citizen scientists working for a project called Einstein@Home, which relies on volunteers to analyze data from the NASA Fermi Gamma-ray Space Telescope (FGST). This project has been responsible for the discovery of 23 pulsars so far.

However, it was this particular discovery that was especially significant. Since it was first discovered, a team led by Frank Schinzel of the National Radio Astronomy Observatory (NRAO) conducted follow-up radio observations using the Karl G. Jansky Very Large Array(VLA) in New Mexico. These showed that the pulsar had a tail of shocked particles and magnetic energy that extended 13 light-years behind it.

Even more interesting was the fact that this tail pointed towards the center of a supernova remnant located 53 light-years behind it (CTB 1). This tail was the result of the pulsar’s rapid motion through interstellar gas, which resulted in shock waves that produce magnetic energy and accelerated particles in its wake. As Shinzel explained in a recent NASA press release:

“Thanks to its narrow dart-like tail and a fortuitous viewing angle, we can trace this pulsar straight back to its birthplace. Further study of this object will help us better understand how these explosions are able to ‘kick’ neutron stars to such high speed.”

Relying on Fermi data, the team was able to measure how quickly and in what direction the pulsar was moving. This was accomplished through a technique known as “pulsar timing”, where gamma-ray flashes that occur with every rotation of the pulsar (in J0002’s case, 8.7 times a second) are used to track motion.

From this, the team determined that J0002 was traveling at a velocity of about 1125 km/s (700 mps) or 4 million km/h (2.5 million mph). In the past, scientists have observed pulsars traveling at high speeds, but at an average velocity that was about five times slower – 240 km/s (150 mps). As Dale Frail (a researcher from the NRAO who was part of the discovery team) explained:

“The explosion debris in the supernova remnant originally expanded faster than the pulsar’s motion. However, the debris was slowed by its encounter with the tenuous material in interstellar space, so the pulsar was able to catch up and overtake it.”

The team also determined that the pulsar would have eventually caught up with the expanding shell created by the supernova. At first, the supernova’s expanding debris would have moved outward faster than J0002, but after about 5000 thousands years, the shell’s interaction with interstellar gas gradually slowed it down. By 10,000 years, which is what astronomers are seeing now, the pulsar was well outside of the shell.

While astronomers have long-known that pulsars can get a kick in speed from the supernova explosions that create them, they remain unclear as to how that happens. A possible explanation is that instabilities in the collapsing star could have produced a dense, slow-moving region of matter that began pulling the neutron star along, gradually accelerating it away from the center of the explosion.

CTB 1, seen here in a deep exposure that highlights visible light from hydrogen gas. Credit and ©: Scott Rosen/NASA/GSFC

“This pulsar is moving fast enough that it eventually will escape our Milky Way Galaxy,” said Frail. “Numerous mechanisms for producing the kick have been proposed. What we see in PSR J0002+6216 supports the idea that hydrodynamic instabilities in the supernova explosion are responsible for the high velocity of this pulsar.”

Looking ahead, the team plans to conduct additional observations using the VLA, the National Science Foundation’s Very Long Baseline Array (VLBA) and NASA’s Chandra X-ray Observatory. These follow-ups will hopefully provide more clues as to how this pulsar picked up so much speed, which could go a long way towards resolving some of the mystery that still surrounds supernovae explosions.

These results were recently shared at the 17th High Energy Astrophysics Division (HEAD) meeting of the American Astronomical Society, which was held from March 17th to 21st in Monterey, California. They are also the subject of a study that is being reviewed for publication in the latest issue of The Astrophysical Journal Letters.

Further Reading: NRAO, NASA

Researchers make coldest quantum gas of molecules

February 21, 2019, National Institute of Standards and Technology

Artist's impression of JILA's record-cold quantum gas of potassium-rubidium molecules. Credit: Steven Burrows/JILA

JILA researchers have made a long-lived, record-cold gas of molecules that follow the wave patterns of quantum mechanics instead of the strictly particle nature of ordinary classical physics. The creation of this gas boosts the odds for advances in fields such as designer chemistry and quantum computing.

As featured on the cover of the Feb. 22 issue of Science, the team produced a gas of potassium-rubidium (KRb) molecules at temperatures as low as 50 nanokelvin (nK). That's 50 billionths of a Kelvin, or just a smidge above absolute zero, the lowest theoretically possible temperature. The molecules are in the lowest-possible energy states, making up what is known as a degenerate Fermi gas.

In a quantum gas, all of the molecules' properties are restricted to specific values, or quantized, like rungs on a ladder or notes on a musical scale. Chilling the gas to the lowest temperatures gives researchers maximum control over the molecules. The two atoms involved are in different classes: Potassium is a fermion (with an odd number of subatomic components called protons and neutrons) and rubidium is a boson (with an even number of subatomic components). The resulting molecules have a Fermi character.

JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. NIST researchers at JILA have been working for years to understand and control ultracold molecules, which are more complex than atoms because they not only have many internal energy levels but also rotate and vibrate. The JILA team made their first molecular gas 10 years ago.

"The basic techniques for making the gas are the same ones we've used before, but we have a few new tricks such as significantly improving the cooling of the atoms, creating more of them in the lowest-energy state," NIST/JILA Fellow Jun Ye said. "This results in a higher conversion efficiency so we get more molecules."

The JILA team produced 100,000 molecules at 250 nK and as many as 25,000 molecules at 50 nK.

Before now, the coldest two-atom molecules were produced in maximum numbers of tens of thousands and at temperatures no lower than a few hundred nanoKelvin. JILA's latest gas temperature record is much lower than (about one-third of) the level where quantum effects start to take over from classical effects, and the molecules last for a few seconds—remarkable longevity, Ye said.

The new gas is the first to get cold and dense enough for the matter waves of these molecules to be longer than distances between them, making them overlap with each other to create a new entity. Scientists call this quantum degeneracy. (Quantum matter can behave as either particles or matter waves, that is, waveform patterns of the probability of a particle's location).

Quantum degeneracy also means an increase in the repulsion among fermionic particles, which tend to be loners anyway, resulting in fewer chemical reactions and a more stable gas. This is the first experiment in which scientists have observed collective quantum effects directly affecting the chemistry of individual molecules, Ye said.

"This is the first quantum degenerate gas of stable molecules in bulk, and the chemical reactions are suppressed—a result that nobody had predicted," Ye said.

The molecules created in this experiment are called polar molecules because they have a positive electric charge at the rubidium atom and a negative charge at the potassium atom. Their interactions vary by direction and can be controlled with electric fields. Polar molecules thus offer more tunable, stronger interactions and additional control "knobs" compared with neutral particles.

These new ultralow temperatures will enable researchers to compare chemical reactions in quantum versus classical environments and study how electric fields affect the polar interactions. Eventual practical benefits could include new chemical processes, new methods for quantum computing using charged molecules as quantum bits, and new precision measurement tools such as molecular clocks.

The process for making the molecules begins with a gas mixture of very cold potassium and rubidium atoms confined by a laser beam. By sweeping a precisely tuned magnetic field across the atoms, scientists create large, weakly bound molecules containing one atom of each type. This technique was pioneered by Ye's colleague, the late Deborah Jin, in her 2003 demonstration of the world's first Fermi condensate.

To convert these relatively fluffy molecules into tightly bound molecules without heating the gas, scientists use two lasers operating at different frequencies—each resonating with a different energy jump in the molecules—to convert the binding energy into light instead of heat. The molecules absorb near-infrared laser light and release red light. In the process, 90 percent of the molecules are converted through an intermediate energy state, to the lowest and most stable energy level.

Fuente: https://phys.org/news/2019-02-coldest-quantum-gas-molecules.html

La NASA da por perdido a su robot marciano Opportunity: no sobrevivió a la gran tormenta de polvo

El 'rover' no da señales desde junio y los intentos por contactar con él han fracasado

Tanto él como su gemelo 'Spirit' cambiaron el conocimiento Marte con sus hallazgos

Sin noticias de 'Opportunity' tras la gigantesca tormenta marciana

Así se convirtió un lago marciano en una montaña de 5.500 metros

 

Los científicos de la NASA no perdían la esperanza pero tampoco eran optimistas. Su vehículo robótico Opportunity no envía ninguna señal desde que el pasado mes de junio tuvo que hacer frente a una gran tormenta de polvo en Marte. La agencia espacial ha hecho a lo largo de estos meses varios intentos para restablecer la comunicación con su rover, que el pasado 24 de enero cumplió 15 años en el planeta rojo.

Pero según han explicado esta tarde los responsables de la misión en una rueda de prensa, su último intento tampoco ha dado resultado así que la NASA ha decidido dar por terminada la misión de su emblemático rover en Marte.

"Opportunity se ha ido pero nos ha dejado su legado", ha declarado el director del Jet Propulsion Laboratory (JPL), Michael Watkins, que ha descrito al rover como "un geólogo robótico". Tanto el director de la NASA, Jim Bridenstine, como el director de Ciencia, Thomas Zurbuchen -que ha definido a Opportunity como"un héroe"-, han preferido considerar esta jornada de despedida como un día para celebrar los logros del icónico robot marciano.

"Hoy es un día triste", comenta a EL MUNDO Alberto G. Fairén, el único científico español que ha formado parte del equipo de Opportunity. "Es uno, por no decir el mejor, de los ingenios mejor diseñados por el ser humano: fue concebido para trabajar en Marte durante tres meses (90 soles) y estuvo trabajando de forma continuada 14 años y 4 meses (desde el 25 de Enero de 2004 hasta el 10 de Junio de 2018)", apunta su colega Jorge Pla-García, uno de los investigadores del Centro de Astrobiología (CAB/CSIC-INTA) que se encarga de la estación meteorológica de Curiosity, el único rover que queda ahora operativo en Marte tras la muerte de Opportunity.

Nubes de hielo de agua formadas cuando se acerca el invierno marciano captadas por 'Opportunity'. NASA

Y es que tanto Opportunity como su rover gemelo Spirit (que quedó atrapado en un cráter en 2009) superaron con creces las expectativas puestas en su misión, denominada MER (de Mars Exploration Rovers). Los dos partieron rumbo a Marte en 2003 (Spirit fue lanzado el 10 de junio y Opportunity, el 7 de julio) y llegaron en enero de 2004. Opportunity, señala García-Pla, "ha sido el vehículo que mayor distancia ha recorrido en un cuerpo extraterrestre en la historia (45,16 kilómetros), superando con creces los 39 kilómetros recorridos por el rover soviético Lunokhod 2 en la Luna".

Hay muchos aspectos en los que los vehículos MER fueron pioneros, como enumera Alberto G. Fairén: "Fueron los primeros robots en mostrarnos muchas cosas que no sabíamos de Marte: que Marte es gris, no rojo, porque llevaban unos pequeños pinceles para limpiar la capa de polvo rojo en las rocas, y debajo del polvo los materiales son grises; fueron los primeros en detectar meteoritos sobre la superficie de otro mundo, en hacer perfiles térmicos de la atmósfera marciana y en cuantificar la erosión eólica sobre diversas superficies; validaron por primera vez in situ los datos obtenidos por varias misiones orbitales y fueron los primeros en determinar la composición y distribución de rocas, minerales y suelos sobre la superficie de Marte".

Pero, sobre todo, subraya Fairén, "fueron los primeros en demostrar que Marte fue un mundo habitable hace miles de millones de años, con agua líquida estable sobre su superficie: Spirit encontró los restos de antiguos manantiales hidrotermales, y Opportunity descubrió las huellas de lagunas de extensión variable que anegaban episódicamente las planicies de Meridiani hace más de 3.500 millones de años. El agua era ácida y posiblemente salada, un entorno ideal para algunos microorganismos extremófilos de la Tierra".

Opportunity demostró además que Marte ha cambiado mucho a lo largo del tiempo y según las zonas, pues tuvo en algunos lugares y momentos aguas ácidas y saladas, y en otras zonas y periodos agua dulce: "Esto hace que su pasado sea mucho más parecido a la Tierra, y mucho más interesante desde un punto de vista astrobiológico", explica el investigador. Clave para llegar a esta conclusión fue el hallazgo de los materiales más antiguos que se han analizado hasta la fecha in situ, unos sedimentos de hace 4.000 millones de años que contienen arcillas.

Durante su aventura marciana, el rover observó nubes de hielo de agua, escarcha sobre una de sus cámaras y fotografió campos de dunas esculpidas por el viento, además de descubrir el mineral jarosita en Marte, que como recuerda García-Pla, se llama así por el lugar dónde se descubrió, el Barranco del Jaroso en Almería. "Por los estudios realizados en la Tierra, sabemos que este mineral sólo se puede formar por la presencia de agua líquida".

ROSALIND FRANKLIN, EL FUTURO ROVER EUROPEO

El 'rover' de la misión ExoMars se llamará Rosalind Franklin ESA/ATG

La nueva generación de vehículos robóticos que toma el relevo de los gemelosOpportunity y Spirit cuenta con instrumentos más sofisticados para explorar el planeta rojo. Con la muerte de Opportunity sólo Curiosity, también de la NASA, está operativo en la actualidad. Desde 2012, explora el cráter Gale.

Para 2020 está previsto el lanzamiento del vehículo robótico de la misión Exomars, que la Agencia Espacial Europea (ESA) está desarrollando junto a Roscosmos, la agencia rusa. Se va a llamar Rosalind Franklin en homenaje a la científica británica que junto a otros colegas descubrió la estructura del ADN, según anunció la semana pasada la ESA.

Franklin (1920-1958) falleció a los 37 años de un cáncer de ovarios, cuatro años antes de que sus colegas Francis Crick, James Watson y Maurice Wilkins ganaran el Premio Nobel en 1962 por sus trabajos sobre el ADN. Aunque los Nobel no se pueden otorgar póstumamente, se criticó que el nombre de Franklin no fuera mencionado durante los discursos. Con los años, su papel en este descubrimiento ha sido reconocido, como refleja la elección de su nombre para bautizar al sofisticado rover marciano.

Su nombre fue sido seleccionado por un panel de expertos a partir de una lista de candidatos propuesta por el público (unas 36.000 personas participaron en el concurso organizado en Reino Unido). "Este nombre nos recuerda que explorar está en los genes humanos. La ciencia está en nuestro ADN y en todo lo que hacemos en la Agencia Espacial Europea", declaró Jan Woerner, director de la ESA.

El rover Rosalind Franklin cuenta con un taladro que le permitirá perforar la superficie marciana hasta los dos metros de profundidad. Con los análisis de las muestras que tome los científicos quieren investigar la presencia de vida en el pasado marciano.

Fuente: https://www.elmundo.es/ciencia-y-salud/ciencia/2019/02/13/5c64155afdddff6b268b46c9.html