Don't panic, but our planet is doomed. It's just going to take a while. Roughly 6 billion years from now, the Earth will probably be vaporized when the dying Sun expands into a red giant and engulfs our planet. But the Earth is just one planet in the solar system, the Sun is just one of hundreds of billions of stars in the galaxy, and there are hundreds of billions of galaxies in the observable universe. What's in store for all of that? How does the universe end?
This was the most-read story on BBC Earth in 2015. Here is another chance to read it. It could happen to anyone. Maybe you're out trying to find a new habitable planet for the human race, or maybe you're just on a long walk and you slip. Whatever the circumstances, at some point we all find ourselves confronted with the age-old question: what happens when you fall into a black hole?
James Joyce described it as snot-green. Lord Byron went for plain old dark blue. Homer's was frequently "wine dark". These literary greats were describing the colour of the sea, and the variation in their prose wasn't just poetic licence.
The recent film The Theory of Everything tells the story of Stephen Hawking, who managed to become a world-famous physicist despite being confined to a wheelchair by a degenerative disease. It's mostly about his relationship with his ex-wife Jane, but it does find a bit of time to explain what Hawking has spent his career doing.
Chris McKay's quest for extraterrestrial life started in 1976, when Viking 1 and 2 landed on Mars. Touching down on Mars for the first time was a big deal, sure, but the then-first-year graduate student was especially excited because the landers found what appeared to be signs of Martian life.
All things must pass. That includes life on Earth, which will surely be wiped out eventually. But how long does it have? The fossil record tells us that life on Earth has lasted at least 3.5 billion years. In that time it has survived being frozen, clobbered by rocks from space, mass poisoning, and even lethal radiation. Clearly, it's hard to completely sterilise the planet. But there's no shortage of potential apocalypses. Which of them will finally render the Earth barren?
There's egg on your face, literally. You tried to juggle some eggs, it all went wrong, and now you've got to shower and change your clothes. Wouldn't it be faster to just un-break the egg? Breaking it only took a few seconds, so why not do that again, but in reverse? Just reassemble the shell and throw the yolk and the white back inside. You'd have a clean face, clean clothes, and no yolk in your hair, like nothing ever happened.
People have wrestled with the mystery of why the universe exists for thousands of years. Pretty much every ancient culture came up with its own creation story - most of them leaving the matter in the hands of the gods - and philosophers have written reams on the subject. But science has had little to say about this ultimate question. However, in recent years a few physicists and cosmologists have started to tackle it.
Popular culture has painted male African elephants as aggressive, anti-social loners. But are they really the solitary beasts we've imagined, or are they just misunderstood?
Raised from birth in a female-dominated world of mothers and maternal helpers, males leave their birth families as teenagers. But what happens next? Where do these independent teenagers go, what do they do, and whom do they learn from?
Over the years researchers have filled in those gaps, and it turns out we have misjudged male African elephants. There's no question that male elephants can be aggressive, hormone–driven killers. But they can also have friendships, and be both leaders and patient teachers.
When it comes to African elephants, "I think people often consider male behaviour less interesting," says Patrick Chiyo of the University of Notre Dame in Indiana. Males are also more difficult to study, he says, because they are not constrained by slow-moving babies and so can range more widely.
Prior to Chiyo's work on males, "there wasn't very much known beyond some studies in Amboseli [in Kenya], but there was a lot assumed," says his former supervisor Susan Alberts of Duke University in Durham, North Carolina.
Many mammals segregate by sex, with males and females occupying different ranges for at least part of the year. This shapes the males' behaviour. They have to be in the right place at the right time to mate, and that may take precedence over more mundane needs like finding food and avoiding predators.
In male African elephants, the drive to reproduce has a strong effect on their social relationships. It causes them to Hulk out.
Cynthia Moss has been studying elephants in Amboseli, Kenya since 1972. "It was already very clear from the work of Iain Douglas-Hamilton that males and females lived very different lives," she says. Much of her work was focused on the matriarchal societies, but she also wanted to get a handle on what the males were doing. So she brought in Joyce Poole to study the males.
They knew that male Asian elephants regularly entered a state called "musth", in which their urge to mate goes into overdrive and they become very aggressive, rather like rutting deer. But this wasn't thought to happen in African elephants.
So when males would occasionally appear in their study area with dribbling penises and exuding a strong smell, at first Moss thought it was an illness. "I called it the green penis disease," she says.
But after seeing some photographs of musth in Asian elephants, Moss and Poole put two and two together, and realized that African elephant males must also be experiencing periodic musth. They described the phenomenon in 1981.
During musth, males are flooded with up to ten times as much testosterone as usual. They have swollen temporal glands: swellings bigger than a grapefruit that stick out behind their eyes. They are also extremely aggressive, and discharge an almost continuous dribble of urine that creates a scent trail as they walk. Moss calls it "a kind of Jekyll and Hyde transformation".
Musth is a form of honest advertising of a male's sexual availability and condition. To females, a musth bull is saying, "I'm in very good condition, I've lived long enough, and I can give you a healthy calf that's going to inherit my good genes, virility, and longevity," says Moss. Their field data showed that musth males were the most active and most likely to mate with the females.
To other bulls, musth is advertising, "I'm in very good shape. I'm surging with aggressive hormones, and I'll kill you if you challenge me." Testosterone-charged musth males sometimes fight to the death. The males jockey for position, moving around each other, culminating in one big clash where they try to tip each other over so that they can stab with their tusks.
Males experience their first musth at about 30 years of age. For young males, it is a short-lived experience lasting a few days or weeks, whereas in older males, musth can last for months. Musth males get an immediate boost in the male pecking order. The hierarchy is typically determined by size, but their heightened hormones mean they may dominate larger non-musth males.
This all seems to paint a picture of male elephants as friendless creatures that squabble whenever they meet. It was once assumed that, after a male elephant has left his birth family, he spent a lot of time alone, or in "social relationships that were not functionally significant," says Alberts. But there is more to it than that.
Elephant males may not form the same kinds of close-knit friendships as female-led groups, but Chiyo and others have shown that male aggregations are far from random.
Although males leave their birth family at an average age of 14, Poole found that they don't leave family life altogether. Instead, they might move off and join another family, or move from family to family. "Up until the age of about 25 they are spending 80% of their time with family groups."
There was a mistaken belief that young males get kicked out of their families. But this came from observations of males that had left their own family and joined another, "where they are not as welcome as they would be with their own family," Poole explains. Matriarchs and coalitions of females were seen chasing young males, and people thought the males were being shunned by their own families.
From a young age, male elephants gravitate towards other males. "They like to hang around with other males in the family, to roughhouse, and then as they get older, they're sparring [play fighting] together," says Poole.
As different families intermingle, males find other males to roughhouse with and play with, and get to know other young males of their age group. As they play, they begin to understand their own strength, build self-knowledge and learn the tactics they will need to use as older males.
Poole once watched a 6-month-old male looking up at a 30-year old who was sampling a puddle of urine left behind by a female. The older male was finding out if the female was fertile, by bringing his urine-dipped trunk tip to the roof of his mouth. There, the specialized vomeronasal organ could detect the key chemical messages. As Poole tells it, "this little male is staring at him, and reaching up and sniffing him, and finally… [the baby] reaches and touches the puddle of urine. So from an early age, they are watching older males."
So which males hang out together? Chiyo and his colleagues have examined male-male relationships in the 1400-strong Amboseli population, where many individuals have been known since birth and are recognizable by their distinctive ears and tusks. They used DNA from dung samples to work out how all the males were related, and noted which males interacted or play-fought.
They found that males were more likely to spend time with relatives, but selectively play-fought with any males close to their own age. Just like females, Chiyo found that social influence appears to increase with age, and older males have a greater influence on group cohesion - although the patterns among males were weaker and more random than bonds between females.
Sometimes male elders teach young ones mischievous habits. This appears to be the case with crop raiding, which males do more often than females, probably because of their elephantine appetites.
Chiyo has also found that the size of groups varies depending on the situation. If they are relatively safe, males only form large groups when food or mating opportunities are abundant. But in areas where humans pose a threat, such as near croplands, males form large groups even when food is scarce, presumably for safety in numbers.
Males also have dominance hierarchies: pecking orders that are strongly affected by the availability of water. Caitlin O'Connell-Rodwell of Utopia Scientific has studied a "boys club" of over 200 males living in Etosha National Park in Namibia. There are no rivers in Etosha, so in dry years the elephants are dependent on permanent water holes for access to drinking water.
At the water hole, elephant males have a surprising repertoire of friendly or "affiliative" behaviours. "It's very striking, it's like bonded males in a bar," says O'Connell-Rodwell. She compares it to backslapping. Friendly males will take their trunks and put them over each other's heads, intertwine their trunks, put their ear over another male's rear or head, and use any way they can to touch each other.
Males are more touchy-feely than the females are during family visits to the water hole. Females are more business-oriented, she says: they "get in there, drink, protect the little ones, and get out."
But the stable hierarchy sometimes breaks down. It exists to minimise conflict over access to resources like water, says O'Connell-Rodwell, so if there is plenty to go around the males can go wild.
O'Connell-Rodwell tracked the males' dominance hierarchies during the dry season over four years; two that were wetter than normal, and two that were drier. During the dry years, everyone followed the rules and adhered to a clearly established pecking order. But in the wet years, there was more aggression, and the hierarchy was nonlinear.
In wet years, "you don't have to kowtow to the don because you can drink anywhere," says O'Connell-Rodwell. As a result, the young bulls don't have as much oversight. Spending less time with their elders, they appear to have more testosterone spikes, and are more aggressive.
It seems clear that the young males are learning from their elders. So when poachers target the largest and oldest males, says O'Connell-Rodwell, it doesn't just shrink the population, but means the loss of an old tusker's influence on the next generation.
One of Poole's favourite male interactions is when older bulls play-fight with younger males. "When an older male wants to spar with a younger male, they will often get down on their knees," says Poole. She says this is a great example of elephant understanding and empathy.
If a 6-ton male elephant can have empathy and get down on his knees to play with the youngsters, perhaps it's time to give a little more credit to the nuanced relationships of these sometimes fierce but often friendly giants.
It began with a few small strange patches of slime, clinging to the rocks of the Heber River in Canada. Within a year, the patches had become thick, blooming mats. Within a few years the mats had grown into a giant brownish-yellow snot. And within a few decades this snot had spread around the world, clogging up rivers as far away as South America, Europe and Australasia.