Type: Prelims Only

A recent hypothesis says that dark matter comprises a large number of compact objects such as primordial black holes.

What are Dark Matters ?

• Astronomical observations suggest that a significant part of the universe is made up of dark matter which interacts with the rest of the universe only through the gravitational pull.
• Many large lab experiments have tried to detect elementary particles that could be candidates for dark matter.
• However, such dark matter particles have not been detected until now.
• Several astronomical observations suggest that all galaxies are embedded in a “halo” of dark matter.
• The “visible” galaxy is like a disc embedded in a dark matter halo that is much larger in size.

What is the recent proposition?

• When the universe was very young, hot and dense – soon after the Big Bang, it must have had quantum fluctuations of its density.
• This, in turn, would have caused some regions to become extremely dense, and therefore, to collapse under their own gravity to form the primordial black holes.
• While we have no conclusive evidence of spotting these objects, some of the binary black hole mergers detected by the LIGO gravitational wave detectors might be primordial black holes.
• The question is open there is good reason to believe that primordial black holes did form in the young universe.

Observing dark matter: Gravitational Lensing

• The paper explores what happens when such objects get in the way of gravitational waves traveling towards the Earth from the distance.
• It invokes a phenomenon called gravitational lensing that is used regularly in astronomy.
• When light travels through space and passes near a massive or compact body – a star, a galaxy or a black hole, for example, the intense gravity of that body may attract the light towards it.
• This causes bending it from its rectilinear (straight line) path.
• This phenomenon is known as gravitational lensing and was first observed by Arthur Eddington in 1919.

How intense are they?

• Massive objects like galaxies can bend light significantly, producing multiple images, this is called strong lensing.
• Lighter objects like stars or black holes bend light less, and this is called micro-lensing.
• A similar lensing can happen to gravitational waves travelling towards the Earth, and this would leave signatures in the detected gravitational waves.
• This can be used to detect the presence, or the existence, of primordial black holes.

Assessing dark matter

• Until now, individual black holes have not marked out these signatures on gravitational waves detected by the LIGO-VIRGO detectors.
• However, if all of the dark matter is made of primordial black holes, they should have produced detectable signatures on the gravitational wave signals.
• The researchers use the non-observation of the lensing signatures to assess what fraction of the dark matter could be made of black holes.

Way ahead

• This provides a new way of constraining the nature of dark matter.
• The study concludes that black holes in the mass range from a hundred to a million solar masses can contribute only up to 50-80% of the dark matter in the universe.
• This is an upper limit and the actual fraction can be much smaller.
• These upper limits will get better and better with more and more observations.

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NASA is set to send its first spacecraft to study Jupiter’s Trojan asteroids to glean new insights into the solar system’s formation 4.5 billion years ago.

Lucy Mission

• Lucy will fly by eight Jupiter asteroids—seven Trojans and one main-belt asteroid — over the next 12 years.
• It is NASA’s first single spacecraft mission in history to explore so many different asteroids.
• Lucy will run on solar power out to 850 million km away from the Sun.
• This makes it the farthest-flung solar-powered spacecraft ever, according to NASA.

What is Jupiter Trojan Asteroids?

• Simply known as Trojans, they are a large group of asteroids that share Jupiter’s orbit around the Sun.
• Thousands of such asteroids exist in a gravitationally stable space.
• The swarms lead and follow the planet Jupiter along its orbit around the Sun.

What exactly are Trojans?

• Lucy’s Trojan destinations are trapped near Jupiter’s Lagrange (L) points, which are gravitationally stable locations — it is where the gravity from the Sun and from Jupiter cancel each other out.
• This means their orbits are stable and the Trojans are trapped in the space between.
• This also means that asteroids are as far away from Jupiter as they are from the Sun.
• Jupiter’s leading and trailing Lagrangian points (L4 and L5) have been stable over the age of the solar system.
• This means that their orbits have accumulated many, many asteroids.
• It makes sense to call a Trojan a co-orbital object, which moves around one of the two stable Lagrangian points.

When and how were they discovered?

• It took many a scientist to understand Trojans, and subsequently, name them so.
• A German astrophotographer in 1906 made an important discovery: An asteroid with a particularly unusual orbit. As Jupiter moved, this asteroid remained ahead of Jupiter.
• It was observed that the asteroid was nearly 60 degrees in front of Jupiter.

Students with engineering background would better understand who Lagrange was. Rest need not care.

Lagrange’s propositions

• This specific position of a particular behavior was predicted by the Italian-French mathematician Joseph-Louis Lagrange over 100 years earlier.
• Lagrange had argued that if a small celestial body is placed at one of two stable points in a planet’s orbit around the Sun (the L4 and L5), the asteroid would remain stationary from the planet’s perspective.
• This is due to the combined gravitational forces of the planet and the Sun.
• Thus, Lagrange’s prediction acquired credibility. More such asteroids were discovered over subsequent months in Jupiter’s Lagrange point L5.

Behind the name: Lucy

• It is the fossil of a hominin that lived 3.2 million years ago.
• She is known to be one of the most famous pre-human fossils in history.
• Nearly 40 percent of the fossilized skeleton of this hominin was discovered in 1974 by a team of paleoanthropologists led by Donald Johanson.
• The name was inspired from the famous Beatles song “Lucy in the Sky With Diamonds,” which Johanson’s team listened to at camp the night of their discovery.

Back2Basics: Lagrange Points

• Lagrange points are positions in space where objects sent there tend to stay put.
• They are named after Italian-French mathematician Josephy-Louis Lagrange.
• At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them.
• These points in space can be used by spacecraft to reduce the fuel consumption needed to remain in position.
• There are five special points where a small mass can orbit in a constant pattern with two larger masses.

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