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How Can We Be So Sure That Mysterious Dark Matter Exists?

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It’s supposed to be the most common form of matter in the universe, but nobody has ever actually seen it.

It has been more than 50 years since astronomers first proposed “dark matter”, which is thought to be the most common form of matter in the universe. Despite this, we have no idea what it is – nobody has directly seen it or produced it in the lab.

So how can scientists be so sure it exists? Should they be? It turns out philosophy can help us answer these questions.

Back in the 1970s, a seminal study by astronomers Vera Rubin and Kent Ford of how our neighbour galaxy Andromeda rotates revealed a surprising inconsistency between theory and observation.

According to our best gravitational theory for these scales – Newton’s laws – stars and gas in a galaxy should rotate slower and slower the further away they are from the galaxy’s centre. That’s because most of the stars will be near the centre, creating a strong gravitational force there.

Rubin and Ford’s result showed that this wasn’t the case. Stars on the outer edge of the galaxy moved about as fast as the stars around its centre.

The idea that the galaxy must be embedded in a large halo of dark matter was basically proposed to explain this anomaly (though others had suggested it previously). This invisible mass interacts with the outer stars through gravity to boost their velocities.

This is only one example of several anomalies in cosmological observations. However, most of these can be equally explained by tweaking the current gravitational laws of Newtonian dynamics and Einstein’s theory of general relativity. Perhaps nature behaves slightly differently on certain scales than these theories predict?

One of the first such theories, developed by Israeli physicist Mordehai Milgrom in 1983, suggested that Newtonian laws may work slightly differently when there’s extremely small acceleration involved, such as at the edge of galaxies. This tweak was perfectly compatible with the observed galactic rotation.

Nevertheless, physicists today overwhelmingly favour the dark matter hypothesis incorporated in the so-called CDM model (Lambda Cold Dark Matter).

This view is so strongly entrenched in physics that is widely referred to as the “standard model of cosmology”. However, if the two competing theories of dark matter and modified gravity can equally explain galactic rotation and other anomalies, one might wonder whether we have good reasons to prefer one over another.

Why does the scientific community have a strong preference for the dark mater explanation over modified gravity? And how can we ever decide which of the two explanations is the correct one?

Philosophy to the rescue

This is an example of what philosophers call “underdetermination of scientific theory” by the available evidence. This describes any situation in which the available evidence may be insufficient to determine what beliefs we should hold in response to it. It is a problem that has puzzled philosophers of science for a long time.

In the case of the strange rotation in galaxies, the data alone cannot determine whether the observed velocities are due to the presence of additional unobservable matter or due to the fact that our current gravitational laws are incorrect.

Scientists therefore look for additional data in other contexts that will eventually settle the question. One such example in favour of dark matter comes from the observations of how matter is distributed in the Bullet cluster of galaxies, which is made up of two colliding galaxies about 3.8 billion light years from Earth.

Another is based on measurements of how light is deflected by (invisible) matter in the cosmic microwave background, the light left over from the big bang. These are often seen as indisputable evidence in favour of dark matter because due Milgrom’s initial theory can’t explain them.

However, following the publication of these results, further theories of modified gravity have been developed during the last decades in order to account for all the observational evidence for dark matter, sometimes with great success. Yet, the dark matter hypothesis still remains the favourite explanation of physicists. Why?

One way to understand it is to employ the philosophical tools of Bayesian confirmation theory. This is a probabilistic framework for estimating the degree to which hypotheses are supported by the available evidence in various contexts.

In the case of two competing hypotheses, what determines the final probability of each hypothesis is the product of the ratio between the initial probabilities of the two hypotheses (before evidence) and the ratio of the probabilities that the evidence appears in each case (called the likelihood ratio).

If we accept that the most sophisticated versions of modified gravity and dark matter theory are equally supported by the evidence, then the likelihood ratio is equal to one. That means the final decision depends on the initial probabilities of these two hypotheses.

Determining what exactly counts as the “initial probability” of a hypothesis, and the possible ways in which such probabilities can be determined, remains one of the most difficult challenges in Bayesian confirmation theory. And it is here where philosophical analysis turns out to be useful.

At the heart of the philosophical literature on this topic lies the question of whether the initial probabilities of scientific hypotheses should be objectively determined based solely on probabilistic laws and rational constraints.

Alternatively, they could involve a number of additional factors, such as psychological considerations (whether scientists are favouring a particular hypothesis based on interest or for sociological or political reasons), background knowledge, the success of a scientific theory in other domains, and so on.

Identifying these factors will ultimately help us understand the reasons why dark matter theory is overwhelmingly favoured by the physics community.

Philosophy cannot ultimately tell us whether astronomers are right or wrong about the existence of dark matter. But it can tell us whether astronomers do indeed have good reasons to believe in it, what these reasons are, and what it would take for modified gravity to become as popular as dark matter.

We still don’t know the exact answers to these questions, but we are working on it. More research in philosophy of science will give us a better verdict.

Antonis Antoniou, PhD candidate in Philosophy of Science, University of Bristol

This article is republished from The Conversation under a Creative Commons license. Read the original article

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Former US Air Force fighter pilot: UFOs use Star Trek-style warp drive

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A former US Air Force fighter pilot asserts that he has deciphered
the method behind the extraordinary maneuvers of UFOs, reports dailystar.co.uk.

Over
the past two decades, numerous military encounters with these enigmatic
crafts have been reported, prompting a significant investigation by the
Pentagon.

One of the most renowned sightings occurred during the
USS Nimitz encounter, where fighter pilots witnessed a UFO descending
from 28,000 feet to just above sea level in less than a second.

This
astonishing movement would imply that the craft reached a staggering
speed of 19,000 miles per hour, a velocity that would be fatal to any
human pilot.

According
to Chris Lehto, a former USAF pilot, the craft exhibits two key
characteristics: it moves without inertia, essentially lacking weight,
and it accelerates at an incredibly rapid pace without affecting its
surroundings.

Image: NATO Allied Air Command/Facebook

Chris
believes that the explanation lies within a technology that seems
straight out of science fiction. He proposes that the answer to the UFO
enigma lies in the Alcubierre Drive, a theoretical interstellar engine
conceptualized by Mexican physicist Miguel Alcubierre in 1994.

The
Alcubierre Drive employs a form of “space warp” technology, reminiscent
of what has been depicted in episodes of Star Trek. By bending space, a
craft inside a “warp bubble” could potentially travel at or even
surpass the speed of light without violating the known laws of physics.

While
the Alcubierre Drive remains a hypothetical concept with challenges to
overcome, Chris notes that the required energy is no longer believed to
be unattainably large.

Patents
filed with the US patent office outline the potential workings of the
drive, as well as another groundbreaking technology theorized by
American aerospace engineer Salvatore Pais.

Pais suggests that
high-powered rotating magnets could theoretically eliminate an object’s
inertia, and he has filed a patent for a starship based on this
principle.

However, Chris maintains skepticism regarding Pais’
theory. He explains that while Pais’s patent applications for the US
Navy attracted attention for their potential energy-related
applications, doubts have been raised about their feasibility. There is
speculation that they may be scams, pseudoscience, or disinformation
intended to mislead adversaries of the United States.

Alternatively,
rival theories propose that the “Tic Tac” UFO is a classified Pentagon
project testing similar advanced technologies discussed by Chris.

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A Mysterious Earth-Like Planet Has Just Appeared in Our Solar System, Scientists Say

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Scientists say they have found evidence of a new Earth-like
planet that has suddenly appeared in our Solar System and is orbiting
the Sun.

Physicists, including those from the National Astronomical Observatory
of Japan, said the planet is likely to be the mysterious ‘Planet Nine’
that was hypothesised to exist in the far outer edges of the Solar
System.

Several studies in the past have suggested there is likely an
undiscovered planet beyond the Kuiper Belt – a stellar disk of materials
such as asteroids, space rocks, comets around the Sun in the outer
Solar System past the orbit of Neptune.

Independent.co.uk reports: In the new research, published recently in The Astronomical Journal, scientists
found that some of the objects in the Kuiper Belt behave in a way
indicative of the presence of a small planet among them.

One such object, they said, is about 500 astronomical units (AU) from
the Sun, where 1 AU is the distance between the Sun and the Earth.

In comparison Neptune is at a distance of 30 AUs from the Sun.

Some of these were also found to have “odd” orbits suggesting they
are being pull by the gravity of a cosmic entity larger than those that
typically influence such objects.

Computer simulations run by the scientists indicate that the most
likely explanation for the observations was another hidden planet in the
Kuiper Belt.

“We predict the existence of an Earth-like planet,” researchers wrote in the study.

“It is plausible that a primordial planetary body could survive in
the distant Kuiper Belt as a Kuiper Belt planet (KBP), as many such
bodies existed in the early solar system,” they added.

If such a planet exists, researchers say it would have a mass about
1.5 to 3 times that of Earth with an inclination of about 30 degrees.

They say the theorised planet’s orbit would likely place it between 250 and 500 AU from the Sun.

Researchers say the discovery of such a planet close to the Kuiper
Belt can unravel new constraints on planet formation and evolution.

“In conclusion, the results of the KBP scenario support the existence
of a yet-undiscovered planet in the far outer solar system,” scientists
noted.

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