What caused the extinction of dinosaurs? This question has been haunting the scientists for a very long time. One of the dominant theories involves our planet’s collision with a massive heavenly body, such as an asteroid or comet, some 65 million years ago. The fact of simultaneous extinction of about 16% of marine and 18% of terrestrial animal families also points the same way. But why of all times did the collision occur at exactly the end of Cretaceous, when the period of intense bombardment of our planet had long ended and explosive development of life began in all its multiple forms? The thing is, in the space beyond the bounds of the Solar System there is a region filled with ice and interstellar material believed (for direct observation of that region has not been made possible yet) to be a home to quite a number of comets and other space objects, which at certain intervals change their trajectories to approach our Earth and other planets.
Oort cloud
This region, which goes under the name of Oort cloud, lies well out of the Solar System, spanning approximately 50 to 100 thousand astronomical units (one unit equals the distance between the Earth and the Sun), which is about a quarter the of distance to Centauri Proxima, the star closest to the Earth. On the whole the cloud is a stable formation – since our planet formed in the geological sense of the term the Earth has not had too many collisions with massive heavenly bodies. The interesting part is, bursts of comet activity have been observed to recur about every 35 million years. Luckily, most of the comets simply fail to reach the Earth surface, torn into pieces by the gravitational fields of the nearby Solar System planets. The individual fragments are not that dangerous, at least on a planetary level. Yet, if you look at the demolition caused by fragments of comet Shoemaker-Levy, which fell on Jupiter in 1994, you will probably agree that a similar cataclysm would have caused very serious consequences for the Earth. But what exactly makes comets leave the Oort cloud and head for the Earth on a convergent trajectory? Two possible explanations have been proposed so far. The first one involves the gravitational pull of an as-yet-undiscovered distant companion star to the Sun. The other involves the oscillations of the gravitational field as the Solar System crosses the disc (plane) of the Milky Way. As has been suggested by Lisa Randall and Matthew Reece of Harvard University in the March issue of Physics World, the gravitational field of only the visible matter of the galactic disc is too small to have much of an effect on the paths of the comets in the Oort cloud.
The true reasons are often hidden from the eye of the observer
In 1933 Fritz Zwicky, an American astronomer of Swiss origin, at the time busy calculating the mass of a galactic cluster in the constellation of Coma by measuring the rotation speed of galaxies (of which there are about 600) – the so called dynamic mass – found he received a value at odds with the one obtained from the luminosity of the stars of the galaxies. The difference was 50-fold! This is how the term “dark matter” came in existence (borrowed by Zwicky from Oort’s work, by the way.) Today astrophysicists believe the visible (that which gives off light) matter to amount to less that one percent of the total mass of the Universe. The dark matter consisting of baryonic (common) material, i.e. planets, stars either defunct or never lit, intergalactic gas, as well as non-baryonic material, i.e. neutrinos and leptons, accounts for another couple of tens of percent. The rest is the so called dark energy, but we shall talk about it later.
The main problem with the dark matter is, it is difficult to detect: electrically neutral, it can only be betrayed by its gravitational pull. Both theory and observations suggest that dark matter mostly concentrates in the halo around our galaxy, much like the Oort cloud around our Solar System. Last year Randall and Reece, together with JiJi Fan and Andrey Katz, hypothesized that some part of the dark matter might partly interact with the common material by exchanging “dark” photons, i.e. the quanta of electromagnetic emission. As a result, the interacting part of the dark matter (about 5% of the total) might form a “shadow” galactic disc in the same plane as our Milky Way. In their work, available as a preprint from arXiv, Randall and Reece show that in the conventional model (common matter in the center, dark matter around the perimeter) the galactic disc has a surface density of about 7 solar masses per square parsec. By comparison, the suggested new model (in which about 5% of the dark matter is in the “shadow” disc) gives the surface density of about 10 solar masses per square parsec, and the “shadow disc” has a thickness of about 10 parsec. Apart from the fact that the new model offers a better understanding of the registered Oort cloud stability fluctuations, such “shadow disc” is quite a realistic catch for the European Space Agency’s Gaia mission. So if the theory is confirmed, the scientific society will be able to better understand the true behavior of comets, including those with power to threaten the existence of life on the Earth.
Source: PhysicsWorld.
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