Placeholder Image

字幕表 動画を再生する

  • [♪ INTRO]

  • Over the years, we've talked a lot about dark matter, and you can't blame us.

  • Even though it exerts gravitational effects, this stuff is invisible in telescopes.

  • And it's found all over the universe, but scientists aren't totally positive what it is.

  • Dark matter is one of the coolest mysteries in astronomy,

  • but it's also made many of you raise your eyebrows.

  • Whenever we bring up dark matter, commenters ask why scientists are so stubbornly positive it exists.

  • Like, why can't they just admit they're wrong about gravity?

  • The truth is, though, scientists have spent decades trying to alter gravitational laws

  • to explain dark matter... without dark matter.

  • And while they have had plenty of successes,

  • a recent paper says they're really likely to be wrong.

  • Like... this likely to be wrong. That's a lot of 9s.

  • Lots of independent evidence points astronomers toward dark matter.

  • The most famous comes from stars and galaxies.

  • Some of them move too quickly, to the point that the gravity from visible matter

  • can't be the only thing holding them together.

  • Computer simulations of the universe's evolution also require dark matter to produce realistic results.

  • And the Cosmic Microwave Background, temperature variations imprinted on the universe by the

  • Big Bang, has patterns we can't make sense of without a bunch of the stuff.

  • Specifically, we need about five times more dark matter than regular matter.

  • Which is the same amount we infer from those other methods.

  • So it's not like astronomers have been chomping at the bit to postulate what this matter is.

  • They have been forced to their current ideas by almost a century of measurements.

  • Even if we keep failing to discover what exactly dark matter is,

  • the case that it's there seems pretty solid.

  • Then again, to make dark matter work, we do have to make some pretty extraordinary claims.

  • Like, we have to conclude that there's

  • five times more invisible stuff out there than what we can see.

  • So some researchers haven't given up on searching for alternate explanations.

  • One of the most famous is an astrophysicist named Mordehai Milgrom.

  • In the 1980s, he wondered just how much you would have to change our ideas about gravity

  • to explain galaxies' behavior without dark matter.

  • To figure it out, he invented the field of Modified Newtonian Dynamics, or MOND,

  • and it's been growing since his initial papers.

  • Currently, gravity is explained by Einstein's spacetime-warping general theory of relativity.

  • Among many other things, it shows that gravity gets

  • continuously and smoothly weaker as you get farther from an object.

  • MOND is a little different.

  • This method tweaks general relativity's mathematical approach to gravity.

  • Near something massive, its predictions are pretty similar,

  • but they're different as you get farther from something.

  • Instead of gravity getting continuously weaker,

  • MOND usually has what's called a fundamental acceleration scale.

  • It's a sort of lingering gravitational effect

  • where you wouldn't really expect one using general relativity.

  • So far, this new method seems like it can explain a lot.

  • Many galaxies' motions fit MOND's predictions just as well as dark matter's, and simulations

  • with MOND have even had success reproducing the universe's large-scale structure.

  • But the method still has plenty of problems.

  • For example, it has trouble recreating those patterns in the Cosmic Microwave Background.

  • And it can't really account for all the different ways we see galaxies behave.

  • Some galaxies act like they have almost no dark matter in them, while others might be

  • 90% dark matter or more, and MOND has trouble reproducing that variety.

  • MOND also has issues with objects like the Bullet Cluster,

  • an object some 3.7 billion light-years away.

  • There, gravitational effects seem to be completely separated from visible matter,

  • as if there's dark matter in one place and matter in another.

  • Some people would argue that general relativity and dark matter have had a decades-long head start,

  • and that researchers will work out the kinks in MOND eventually.

  • But others aren't so confident.

  • Earlier this year, a team of astronomers searched for the fundamental acceleration scale

  • by looking at the rotation rates of almost 200 galaxies.

  • And they found that MOND doesn't fit thedata.

  • No matter which specific model you're using, MOND would predict the

  • fundamental acceleration scale should be the same throughout the universe.

  • But this team found that different galaxies required a different scale,

  • which kind of throws a wrench in things.

  • Of course, nothing in science is 100% certain.

  • But the team's analysis says that the chance that MOND is still right is pretty slim.

  • Like, about 1 in a hundred billion trillion, a number with 23 zeros in it.

  • Some scientists have disputed that extreme statement,

  • saying there's far more uncertainty than the new paper claims.

  • But possible is awfully far from 1 in a hundred billion trillion.

  • Now, this all might have been the death of MOND once and for all,

  • if another paper hadn't come out a few weeks later.

  • In it, a different group studied almost all of the exact same galaxies,

  • but they analyzed the data in a slightly different way.

  • And they found clear evidence of a fundamental acceleration scale that worked for all of them.

  • It's not obvious what caused this difference, but it does mean that MOND isn't dead yet.

  • At least for now, though, the majority of astronomers and cosmologists do think dark

  • matter is still the right approach, and they have decades of good evidence to support that.

  • After all, on scales bigger than individual galaxies,

  • MOND repeatedly fails while dark matter repeatedly succeeds.

  • Researchers will keep looking into alternate explanations, but, at least for now,

  • no other idea is anywhere near as successful as dark matter is.

  • So we'll just have to keep looking.

  • Thanks for watching this episode of SciShow Space, and thanks to all the commenters

  • who asked questions and inspired this episode!

  • If you have a pressing space question or fact you think is really cool,

  • go ahead and leave it in the comments.

  • We'll do our best to check them all out.

  • If you want to make sure we see your question, though, you can go to patreon.com/scishow.

  • [♪ OUTRO]

[♪ INTRO]

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B1 中級

ダークマターオルタナティブの残念な状態 (The Sorry State of Dark Matter Alternatives)

  • 0 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語