Plot Twist: There’s No Dark Matter. Our Theory of Gravity is Broken

It has been 90 years since the concept of dark matter was introduced in astronomy. It lies at the heart of the most successful cosmology model, the LCDM model. However, even after a century, astronomers struggle to know the true nature of dark matter. Even the Standard Model of particle physics is silent about it. Several experiments on dark matter have returned empty-handed. This raises a critical question: What if there's no dark matter? What if there's a fundamental flaw in our understanding of gravity itself?

Some physicists are trying to develop a new theory of gravity that explains galaxy rotation curves without needing dark matter. And the latest study shows that one such modified theory called AQUAL can explain galaxy rotation curves better than the LCDM model.

The 47th episode of the Sunday Discovery Series explains the discovery and the paper in detail.

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Created By: Rishabh Nakra and Shreejaya Karantha
Narrated By: Jeffrey Smith

The Secrets of the Universe on the internet:

[caseypayne]

honestly I always kinda felt like this was the case...dark matter is kind of just a placeholder concept until we figure out what's really going on there

[kgreen]

Negative information is the key here I think. Knowing what something isn't can often lead one to discovering what something actually is

[craig]

Remember how complicated the orbits of planets were... until the sun was put at the centre of the solar system? Are we missing something like that, and when discovered will explain a lot of things without having to add cosmological constants, dark matter and the like? Probably.

[CitiesTurnedToDust]

To me, the big flaw with these theories is that I've been reading about galaxies we've found which act as if they have no dark matter at all, and others which seems to be made of nearly all dark matter. So that I don't see how this idea can stand up against those observations.

[robertsteinbach]

The ultimate criteria for a new model being used is: does it more accurately predict what we would see in the aspect you need better than the previous model.
The process to go from an Earth centric to a Sun centric model started with Copernicus and ended with Kepler and Newton because the new model became better than the old model.
The same thing will happen but it will take time and patience to get a new model right.

[malcolmabram]

It is easy to look at pictures of galaxies, yet fail to appreciate how utterly vast they are. In our own galaxy, the Milky Way, the far edge is over 60, 00 light years away, i.e. what we see is what it looked like 60, 000 years ago, or 58, 000 BC. I am intrigued that the rotation of such a vast object can actually be observed, even over a period of several years.

[ThePirateParrot]

It's not really a plot twist we know our model of gravity is incomplete. I still like the additional dimension based theories. A dimension beyond our ability to perceive is just a neat idea.

[JokuPalloKala]

Cool video, but one thing to keep in mind: Modifier gravity is specifically *designed* to match rotation curves, so it's not exactly strong evidence that a theory designed to explain something very well explains that one thing very well. I'd be more curious on the predictions and explanations on everything else.

[johnnytoobad]

I never felt comfortable with the current theories of "dark matter" and "dark energy". It's just scientific double speak for "...we don't know", so let's make something up so that we can plug "fudge factors" into the models we have. A technique learned from Tycho Brahe.

[MrEcted]

"dark matter" has never sat right with me. I'm just an armchair type so I don't know much about anything, but I have always wondered if there's a very simple fundamental misunderstanding that has inferred its existence when in fact it doesn't really exist.

It's similar to quantum mechanics/particle physics for me. Big elaborate interpretations like many-worlds or Copenhagen are all the rage, but in the end I think it's going to be something more "boring", like pilot wave (de Broglie/Bohm), it's just that the idea of many-worlds, or all these crazy probability states that collapse when observed are more enticing because it gets our imaginations going. Of course pilot-wave has its own set of problems (non-locality for instance), but I feel like, yet again, our imaginations are tripping us up.

I'd love to be wrong though (and I usually am), I love a good mystery!

[eerohughes]

If it can't account for gravitational lensing or how the galaxies hold together than it has a Major problem. I don't feel this theory holds up.

[randallporter]

I've always believed our scientists made up Dark matter/energy because they didn't fully understand Gravity.

[neomorphicduck]

They are saying "We tuned a model to get better results on one aspect, but cannot replicate any of the results in another aspect" I hardly think this means we should throw out Dark Matter at this point. We definitely need a way to explain gravitational lensing before we can trust these tuned models.

[Pokemaster-wggx]

I love how the difference is basically
Guy 1: *y’know, we don’t know everything and something currently unseeable is currently unseeable*
Guy 2: *Bullshit, lemme just tweak some old things and invent stuff so i have an excuse to not confront not being able to understand everything*

[glynbrain]

If photons gradually lengthened their wavelength per unit distance travelled (very gradually), it would create a second form of red shift (in addition to Doppler), and account for the universe 'appearing' to expand, and if the wave lengthened enough it would be virtually undetectable - but it would still exist.

[wolfthorn]

I use "dark matter all the time."
When I am missing an answer I just say "dark matter, or even dark energy."
For example 2+2=348, People say no, wrong.
I say.... "did you account for the dark matter?"
Of course you are right they say.

[Jamex]

You don't discover a theory, you theorize it

[michaelkahn]

Alternative Explanation of Dark Matter and Dark Energy - Newly proposed model of Universe can explain both of Dark Matter and Dark Energy

Einstein’s theory of General Relativity states that spacetime is curved by the presence of mass. This curvature influences the motion other objects with mass and gives rise to gravitation. Thus, gravity is a result of geometric features in spacetime.

However, we also observe gravitational effects – curvature of spacetime – in areas without any detectable mass. This has given rise to the concept of dark matter, which is matter that does not interact in any detectable way with normal matter, except through gravity. So, there is some large quantity of dark matter scattered throughout the universe, which curves spacetime and causes gravitational effects just like normal matter, but we cannot see or detect it with any known method.

An alternative theory to the identity of dark matter is proposed – it is not matter at all, but rather an intrinsic curvature of spacetime. In other words, spacetime is not naturally flat. Even in the absence of matter, we observe some inherent curvature of spacetime.

So, the question is now – why is spacetime naturally curved? Why is it not flat in the absence of mass?

The universe is 4-dimensional, with 3 spatial dimensions and one dimension in time. Rather than consider time as a linear dimension, we can consider it as a radial one. Therefore, rather than describing the universe with a Cartesian coordinate system, we describe it with a 4-dimensional spherical coordinate system – 3 angular coordinates, φ1, φ2, φ3, and one radial coordinate in time, t. We live on the 3-dimensional surface of a 4-dimensional bubble which is expanding radially in time. Thus, the Big Bang represents t=0, the beginning of time.

The crucial point is that the expansion of the universe is not homogeneous in all directions. The expansion rate at one point on the bubble’s surface may differ slightly from another point near it. The universe is only roughly spherical in 4 dimensions, the same way that the Earth is only roughly spherical in 3 dimensions. The same way we observe local mountains and valleys on the surface of Earth, we observe local “mountains” and “valleys” on the surface of the universe bubble. The inhomogeneity of the expansion of the universe has given rise to natural curvature of spacetime. This natural curvature causes the phenomenon of “dark matter”.

“Valleys” in spacetime pull matter in, similarly to the warping of spacetime of massive objects. So “dark matter” is really “valleys” in spacetime that are expanding slower than the regions surrounding it. These valleys tend to pull matter in and create planets, stars, and galaxies – regions of space with higher-than-average densities of mass. Conversely, “mountains” in spacetime will repel matter away, an “anti-gravitational” effect, which gives rise to cosmic voids in space where we observe no matter.

Each point on the surface of the universe bubble traces out a time arrow in 4-dimensional space, perpendicular to the surface. These time arrows are not parallel to each other since the universe is not flat. This causes points to have nonzero relative velocity away from each other. It is generally accepted that the universe is expanding faster than observable energy can explain, and this is expansion is believe to be still accelerating. The “missing” energy required to explain these observations has given rise to the theory of dark energy. The time dilation caused by non-parallel time arrows can be proposed as an explanation for dark energy. Alternatively, dark energy is real energy coming from potential energy gradients caused by non-parallel time arrows.

As a sanity check, we can calculate the expansion rate of the universe based on the universe bubble model. Since the radius of the universe bubble is expanding at the speed of light in the time direction, it increases at 1 light second per second. Therefore, the “circumference” of the 3-dimensional surface increases by 2π light seconds per second, or about 1.88*10^6 km/s. This expansion is distributed equally across the 3-dimensional surface, so the actual observed expansion rate is proportional to the distance from the observer. At present, the age of the universe is estimated to be 13.8 billion years, so the radius of the universe bubble is 13.8 billion light years, or about 4233 megaparsecs (3.26 million light years to 1 Mpc). Thus, we can calculate the expansion rate of the universe, per megaparsec from the observer, as:

Expansion rate =

[HEROlCS]

Instead of modifying our current theory of gravity can we instead modify our current understanding of particle mass? Maybe an electron in Andromeda doesn't weigh the same as an electron on Earth, maybe particles shed mass as they travel for 13 billion years, maybe particles naturally decay with an extremely long half life?

[nsbdnow]

I think it's so fun to think we might be totally wrong in our understanding and are about to move forward in a big way. Science!