Astronomy

The earliest black holes in the Universe called primordial black holes (PBHs), are strong contenders to help explain why the Universe is heavier than it looks. There’s only one problem: these miniature monsters haven’t exactly been observed—yet. But, when astronomers do find them, they might turn out to be part of the Universe’s dark matter component.

Primordial black holes are one of several types of highly massive objects thought to exist in the Universe. We already know about stellar-mass black holes. They form during the deaths of hugely massive stars and generally end up containing up to dozens of solar masses. Then there are the supermassive black holes, embedded in the hearts of most galaxies. They sequester up to millions of solar masses.

The intermediate-mass black holes occupy the middle of the “black hole” spectrum. They’re another hot topic in black hole research circles. Appropriately enough, the masses of these black holes are between their stellar and supermassive counterparts. All these types of massive objects can collide with each other to grow bigger black holes. That generates gravitational waves that can be detected. The “ping” of each gravitational wave tells scientists a great deal about the objects colliding, including their masses.

How we might discover primordial black holes and help solve the dark matter mystery. Credit: ESA Understanding Primordial Black Holes in Context of Cosmic History

While astronomers search for PHBs, others are looking to explain why they might be part of the dark matter component of the Universe. In addition, they could explain the origin of binary black holes detected in gravitational wave observations.

A team of researchers at the University of Tokyo examined the “problem” of PBHs. Their work suggests that there should be far fewer of these objects than current models show. But, nobody knows how many existed back then. So, astronomers search them out using gravitational wave observatories. Their discovery should open a window on conditions in the early Universe when PBH formed.

These miniature ones are fascinating to think about. “Many researchers feel they are a strong candidate for dark matter, but there would need to be plenty of them to satisfy that theory,” said graduate student and team member Jason Kristiano. “They are interesting for other reasons too, as since the recent innovation of gravitational wave astronomy, there have been discoveries of binary black hole mergers, which can be explained if PBHs exist in large numbers. But despite these strong reasons for their expected abundance, we have not seen any directly, and now we have a model which should explain why this is the case.”

Modeling the Existence of Primordial Black Holes

The big question about PHBs: do (or did) they exist? And, can they be part of the dark matter component of the Universe? To answer that, Kristiano and his advisor Jun’ichi Yokoyama, searched through models of PBH formation. The best ones do not agree with the observed conditions of the leftover light fingerprint of the Big Bang. That’s called the cosmic microwave background (CMB). This is important, since PBHs formed in very early epochs of cosmic history, soon after the Big Bang. So, the team used the best model of PBH formation and applied quantum field theory to bring the model into alignment with reality.

Yokoyama explained the background behind their work. “At the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded that by 25 orders of magnitude. At that time, waves traveling through this tiny space could have had relatively large amplitudes but very short wavelengths. What we have found is that these tiny but strong waves can translate to otherwise inexplicable amplification of much longer waves we see in the present CMB,” said Yokoyama.

“We believe this is due to occasional instances of coherence between these early short waves, which can be explained using quantum field theory, the most robust theory we have to describe everyday phenomena such as photons or electrons. While individual short waves would be relatively powerless, coherent groups would have the power to reshape waves much larger than themselves. This is a rare instance of where a theory of something at one extreme scale seems to explain something at the opposite end of the scale.”

From Fluctuations to Miniature Black Holes

Those early small-scale fluctuations Yokohama describes affect some of the larger-scale fluctuations in the cosmic microwave background. Researchers can use measurements of wavelengths in the CMB to constrain the extent of corresponding wavelengths in the early Universe. That also puts some limits on any other phenomena that rely on the shorter, stronger wavelengths. And this is where the PBHs come back in.

“It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes,” said Kristiano. “Our study suggests there should be far fewer PBHs than would be needed if they are indeed a strong candidate for dark matter or gravitational wave events.”

The next step relies on gravitational wave observatories and other types of observations. LIGO in the U.S., Virgo in Italy and KAGRA in Japan, are cooperating in observations aimed at finding the first PHBs. The results should help refine the ideas from Yokoyama’s team about PHBs and dark matter.

For More Information

The Case of the Missing Black Holes
Constraining Primordial Black Hole Formation from Single-Field Inflation
Note on the Bispectrum and One-loop corrections in Single-field Inflation with Primordial Black Hole Formation

The post Where are All the Primordial Black Holes? appeared first on Universe Today.

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Astronomy is a profession that, so far, has only been done at night, at least on Earth. Light from the Sun overwhelms any light from other stars, making it impractical for both professional and amateur astronomers to look at the stars during daytime. There are several disadvantages to this, not the least of which is that many potentially exciting parts of the sky aren’t visible at all for large chunks of the year as they pass too close to the Sun. To solve this, a team from Macquarie University, led by graduate student Sarah Caddy, developed a multi-camera system for a local telescope that allows them to observe during daytime.

The University has a system known as the Huntsman Telescope, named after the famous Australian spider species. Its design was inspired by the Dragonfly Telescope Array, initially designed by researchers at the University of Toronto and Yale, among other institutions. Both telescopes feature an array of 10 telephoto lenses from Canon, the camera manufacturer, arranged in a honeycomb pattern.

Typically, the telescope is used for nighttime astronomy at the Siding Spring Observatory, about a seven-hour drive from Sydney. However, Ms. Caddy thought it could do better and potentially continue observations during the day.

An image of Betelgeuse during the day using the Huntsman Telescope.
Credit – Macquarie University

They originally tested their ideas, which focused on a number of broadband filters and a single-lens test version of the Huntsman telescope. This allowed them to optimize things like exposure times and timing and show a proof of concept that they then wrote up in a paper in the Publications of the Astronomical Society of Australia. 

In particular, Ms. Caddy and her colleagues are excited about several use cases. One is tracking particular stars that might soon undergo an exciting event. Betelgeuse comes to mind, as astronomers expect it to undergo a supernova sometime “soon,” though soon in astronomical terms could mean anywhere between tomorrow and 10 million years from now. If Betelgeuse happens to be on the other side of the Sun when it goes supernova, without daylight astronomy, there would be months of a gap where we would miss out on collecting data on the supernova that happened nearest to us in recorded history, and astronomers everywhere would be frustrated.

This is exactly why the Huntsman team used a daytime image of Betelgeuse as part of their proof of concept. While it might not look like a typical image of the star that is 650 light years away, the fact that it is visible at all during the daytime is striking.

Betelgeuse is one of the most interesting stars in the sky – a potential supernova that goes through occasional dimming periods, as Fraser explains.

Another use case is the tracking of satellites. As the orbital space around Earth becomes increasingly crowded, there’s a higher likelihood that satellites will begin colliding, which could eventually result in something as severe as Kessler syndrome, which we’ve discussed before here at UT. Unfortunately, astronomers can only track satellites during the night, so if one of their orbits happens to shift for some reason during a day cycle, it would be impossible for them to suggest changes to the orbital paths of other satellites that are close by.

Unless you have daytime astronomy, which allows you to track satellites during the day, there’s a significantly decreased risk of two running into each other unexpectedly. This data can be combined with radar readings to help avoid catastrophic collisions, no matter how crowded orbital space gets.

This proof of concept is a step toward making those observations a reality. As it is more fully tested, the southern sky will become much more accessible, and it could pave the way for other daytime astronomy projects in other parts of the world.

Learn More:
Macquarie University – Stargazing in broad daylight: How a multi-lens telescope is changing astronomy
Caddy, Spitler & Ellis – An Optical Daytime Astronomy Pathfinder for the Huntsman Telescope
UT – Astro-Challenge: Adventures in Daytime Astronomy
UT – Why Can We See the Moon During the Day?

Lead Image:
Macquarie’s Huntsman Telescope can potentially observe space during the day.
Credit – Macquarie University

The post A New Telescope Can Observe Even in Broad Daylight appeared first on Universe Today.

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