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Invisible matter matters – The Varsity – Varsity

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ARTHUR DENNYSON HAMDANI/THEVARSITY

Theories of galaxy formation and dark matter

A galaxy is a huge collection of stars, dust, and gas held together by gravity and moving through space together. Our solar system, like many others, orbits around the center of a galaxy — in our case, the Milky Way. 

So, how did this happen? Why do solar systems exist in these structures rather than in isolation? In other words, what forms galaxies? 

Theories of galaxy formation

Astrophysicists are still researching galaxy formation to this day. Some theories posit that following the Big Bang — the theory that the universe started and expanded from a single intensely dense point — gravity pulled matter to create dense clumps that grouped together. The clumps that did not dissipate because  other opposing forces grew larger, attracting more matter around them. 

Eventually, high amounts of the clumped matter collapsed under their own gravity. These clumps formed within the first few hundred thousand years of the universe’s existence, developing into the first proto-galaxies that we see today. 

A 1962 paper by astrophysicists O. J. Eggen, D. Lynden-Bell, and A. R. Sandage suggested that the mass of proto-galaxies was comparable to the mass of today’s galaxies. At this size, the clumps of matter collapse under their own gravity and become proto-galaxies, which later develop into mature galaxies. This is known as the top-down or Monolithic Collapse model.

However, in 1984, physicists J. R. Primack, G. Blumenthal, and S. M. Fabera proposed the Lambda Cold Dark Matter model, which builds on a bottom-up model that opposes the top-down model. It instead states that proto-galaxies grew into mature galaxies through galactic mergers or the collision of two or more galaxies. The Lambda Cold Dark Matter model adds cold — or slow-moving — dark matter to the equation of galaxy formation. Dark matter is an invisible form of matter that does not interact with light. It does, however, have gravitational effects on galaxies and galaxy clusters. It is through its gravitational influence, then, that astrophysicists measure dark matter and why they think dark matter exists. 

The Lambda Cold Dark Matter model explains the initial clumping of matter after the Big Bang, arguing that the pronounced variations in dark matter density throughout the universe initially clumped together and collapsed under their own gravity, attracting visible matter to form visible proto-galaxies. The model then continues with the bottom-up model of proto-galaxy maturation through galactic mergers. 

Physicists prefer this model, and it is more supported by observational data, such as evidence of mergers that astronomers can observe by viewing distant galaxies in the process of merging, dwarf galaxies near larger galaxies that are at the beginning of merging, or even faint tails of gas associated with galaxies that are left-over indications of past mergers. 

But if dark matter is invisible, how can physicists even study it to develop a model that includes it in galaxy formation?

Dark matter 

Dark matter accounts for around 85 per cent of the total matter in the universe or over five times more than visible matter. 

Each galaxy has its own ‘halo’ of dark matter around it that has more mass than all the other matter in that galaxy put together. This dark matter halo influences the movement of that galaxy. Furthermore, the gravitational interactions of the halos of different galaxies contribute to driving the growth and structure of galaxies. 

The existence of this halo of dark matter can be seen through an effect called gravitational lensing. Massive objects bend space-time, the fabric of the universe, and that warp also causes light to bend around sources of high mass. This causes distortion and magnification of light from objects behind and in the same line of sight as the mass, causing space-time bending. 

Galaxies bend space-time and cause the distortion and magnification of light from other galaxies further back. Physicists use this level of distortion to calculate the amount of mass in a galaxy or a cluster of galaxies, as well as its location. Astrophysicists concluded that dark matter must exist to account for the mass required to cause these distortions. 

The Chandra X-Ray Observatory obtained evidence for dark matter by studying a particular cluster of galaxies called the Bullet cluster

A previous high-energy collision between two large galaxies formed the Bullet cluster around 3.8 billion light years away from Earth. Because dark matter does not directly interact with itself or with other matter like gas, it was not impacted in the collision, whereas other visible matter like gas was. This caused a separation between dark matter and visible matter in the cluster. Optical imaging of the cluster shows this separation of mass and how nearly all the matter in the cluster is dark matter. This separation of matter allows us to avoid conflating dark matter and visible matter – it allows us to understand that dark matter is a separate type of matter from visible matter. 

So, galaxies and how they behave and form largely depend on dark matter interactions, and there is still a lot more to learn about them. Astrophysicists worldwide are still intensely researching questions like what dark matter is made of, what causes supermassive black holes in the middle of galaxies, and many more. In fact, the Dunlap Institute for Astronomy and Astrophysics at U of T is one of the research facilities studying these topics and the many questions they raise.

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The body of a Ugandan Olympic athlete who was set on fire by her partner is received by family

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NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.

Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.

She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.

Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.

“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.

It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.

Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.

Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.

Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.

“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.

He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.

Kenya’s high rates of violence against women have prompted marches by ordinary citizens in towns and cities this year.

Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.

The Canadian Press. All rights reserved.

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The ancient jar smashed by a 4-year-old is back on display at an Israeli museum after repair

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TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.

Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.

Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.

The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.

The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.

It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.

Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.

Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.

Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.

Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.

Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.

The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”

The Canadian Press. All rights reserved.

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B.C. sets up a panel on bear deaths, will review conservation officer training

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VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.

Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”

Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.

“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.

B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.

Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.

Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.

Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.

“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”

Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”

“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.

Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.

“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.

This report by The Canadian Press was first published Sept. 9, 2024.

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