Dark Matter

Introduction

Dark Matter, according to hypothesis, is a kind of matter that accounts for a large part of the total mass in the universe. It cannot be seen with telescopes, simply because it does not emit or absorb light or any electromagnetic radiation. Its existence is inferred from its gravitational effects on visible matter, radiation and the large-scale fabric of the universe. Dark matter is estimated to constitute more than 90% of the matter in the universe.

               

The dark matter hypothesis arises from calculations concerning the Big Bang theory. There is insufficient observed matter to account for the current stability of the universe. If the Big Bang happened as postulated, and the universe contained only observable matter, then it should by now have spread itself into complete dissociation. Therefore, astrophysicists propose that there must be much greater gravity caused by matter, which cannot be detected by any current method. This is the dark matter proposed, and Big Bang astrophysicists assume that 84% of all the matter in the universe is dark matter. Scientists know dark matter exists by its gravitational pull but, unlike regular matter and antimatter, it's so far been undetectable. Regular matter accounts for about 4 percent of the universe's mass, and dark matter makes up about 25 percent. The rest is dark energy, which is also a mystery.

Case Study

What's regarded as the world's most sensitive dark matter detector was lowered into a 70,000-gallon water tank nearly a mile (1.6 kilometers) beneath the earth's surface, shrouding it in enough insulation to isolate dark matter from the cosmic radiation that makes it impossible to detect above ground.

The search in South Dakota began in 2003 after the Homestake Gold Mine in the Black Hills' Lead, South Dakota, shuttered for good. Scientists came to know about the site, and in July, after years of fundraising and planning, the LUX detector was moved into the Sanford Underground Research Facility, 4,850 feet (1,478 meters) below the earth's surface. It took two days to ease the phone booth-sized detector down the once-filthy shaft and walkways that originally opened for mining in 1876 during the Black Hills Gold Rush. There, the device was further insulated from cosmic radiation by being submerged in water that's run through reverse osmosis filters to de-ionize and clean it.

Scientists are currently working to finish the plumbing needed to keep the xenon as clean as possible. The xenon, in both liquid and gas form, will fill the detector and be continuously circulated through a purifier that works much like a dialysis machine, pulling the substance out to remove impurities before pushing it back into the detector.

Bibliography: Dark matter detector nearing activation in US mine, Amber Hunt.

Feeling Heliocentric!

Introduction

T

he Sun is an ordinary star. To us on Earth, it is of crucial importance since no life could exist without it, but it is simply one of billions of stars in the Milky Way Galaxy, itself one of billions of galaxies in the universe. For a star, the Sun is below average size- some astronomers classify it as a “yellow dwarf”. Yet it is massive when compared to the planets. The Sun contains more than 99% of all matter in the Solar System. Its diameter of 1,392,684 kilometres is more than 100 times that of Earth.

According to Stellar Classification, the Sun is a G2V star and is informally designated as a yellow dwarf. G2 indicates its surface temperature of 5778K and V indicates that the Sun, like most stars, is a main-sequence star which generates energy by nuclear fusion of hydrogen nuclei to helium.  In its core, the Sun fuses 620 million metric tonnes of hydrogen each second.

The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves from perihelion in January to aphelion in July.

Internal Layers

At the centre of the Sun is the core, a region of incredible pressure (200 billion times that on the Earth’s surface) and intense heat- about 15 million degrees Celsius. This is the Sun’s nuclear furnace, where the energy that keeps it shining is released. Energy from the fusion reaction flows out from the radiative zone to the connective zone. Here, in a continuous cycle, hot gas bubbles up to the surface before sinking down to be reheated again.

The picture below shows all the layers of the Sun.

An illustration of the structure of the Sun:
1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence

Surface of the Sun

The Sun’s outer shell, the Photosphere, is only about 500km thick and, at 5500 degrees, much “cooler” than at the core. It is in a state of constant motion, like water in a boiling kettle. Hundreds of thousands of flaming gas jets, called spicules, leap up to 10,000km into the Sun’s atmosphere, known as the Chromosphere.

Invisible lines of magnetic force that twist around the Sun’s globe are the cause of many extraordinary features. Huge arches of fire, called Prominences, can be held up above the Sun by magnetism. Flares, sudden, massive explosions of energy, burst forth when the magnetic field shifts. Where the magnetic field lines erupt through the photosphere, there are dark, cooler areas known as sunspots. Beyond the chronosphere lies the corona, the Sun’s hot, shimmering outer atmosphere. This is visible from the Earth only during Total solar Eclipses.

Fate of the Sun

About 7 billion years from now, the hydrogen that the Sun uses as furl to create energy will start to run out. Eventually the Sun will balloon into a Red Giant, engulfing Mercury and perhaps Venus too. This is what Earth’s landscape may look like when this happens. Its oceans and atmosphere have gone and its rocky surface is melting in temperatures of 1500 degrees Celsius.

                        

A White Dwarf

The Sun will eventually shed its outer layers into space. All that will remain of the Sun is a White Dwarf, before it eventually cools and wastes away.

Time travel: The fine line between fiction and reality

I read an interesting article in the newspaper a few days ago which got me thinking. I would like to quote this article here so that you understand just how easily a single action can change the course of history.

“On October 14, 1914, a 27-year-old Private Henry Tandey of Warwickshire had a German soldier in his line of fire. The German soldier was wounded and did not even attempt to raise his own rifle. Tandey chose not to shoot. The German soldier saw him lower his rifle and nodded his thanks before wandering off.

This man was Adolf Hitler.”

As you can see, had this Private shot him, history would have taken another course altogether. But is it possible to somehow change what happened? Is it possible to somehow tell Pvt. Tandey that no good deed goes unpunished?

 Well, sadly, the answer is no. For several decades, scientists have been asked whether time travel to the past is possible. But the answer has remained negative. This is because of three reasons-

• ·        If one intends to travel back in time to a period where his younger self exists, it violates several key principles of Modern Physics. The same matter cannot exist in the same time frame at the same time.
• ·        Secondly, even if somehow the previous point is dodged and the time traveler manages to change something in his past, the event will take place no matter what. In layman’s terms, “It is supposed to happen.”

       What this means is that even IF Hitler had been KIA in 1914, someone else would have risen to take his place decades later. Someone, perhaps Joseph Goebbels (Propaganda Minister) or Himmler or Goring would have been one of the key architects of World War II. This is of course speculation but the fact is- an event which HAS already happened, will continue to happen no matter what change is introduced. Obviously, the basis of the second point is merely a hypothesis because the first point ALWAYS holds true. Time travel to the past is impossible.

• ·        Thirdly, if one intends to travel back in time to a period much before he was born, this would in effect mean- the reaction preceded the action. Simply put, the man existed before he was born. This again, is impossible.

It is for this reason that whenever we talk about Time Travel, we talk about travelling to the future and nothing else because it is physically possible. The only thing we CAN do with the past is observe it, from another frame. If we could somehow travel to a planet millions of light years away from Earth and then observe every tiny detail on Earth from there, we would see Earth as it was millions of years ago.

Types of wormholes

Based on the solutions obtained from the equations of General Relativity, there are two types of wormholes:

1.     The Schwarzschild Wormhole

The first type of solution would be present in the Schwarzschild metric having an eternal black hole, but it was found that this type of wormhole would collapse and destabilize too quickly for anything to cross through from one end to the other. The characteristics of this type of wormhole are very complicated to understand and therefore, I feel, are beyond the scope of this blog. 

2.     The Traversable Wormhole

Wormholes which could actually be crossed are known as Traversable Wormholes. These can connect and thus allow travel from one part of the universe to another or connect two universes. These can be created by holding the “throat” of a Schwarzschild wormhole open with exotic matter. Exotic matter is matter that has negative energy density.

The impossibility of faster-than-light relative speed only applies locally. Wormholes allow superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time. While traveling through a wormhole, subluminal (slower-than-light) speeds are used. If two points are connected by a wormhole, the time taken to traverse it would be less than the time it would take a light beam to make the journey if it took a path through the space outside the wormhole. However, a light beam traveling through the wormhole would always beat the traveler. As an analogy, running around to the opposite side of a mountain at maximum speed may take longer than walking through a tunnel crossing it.

It is these wormholes that feature in sci-fi movies.

Introducing Wormholes

M

y last two posts were all about black holes and the so-called “applications” they may have. One of the central applications of a black hole is a Wormhole. Several physicists around the globe challenge the existence of wormholes simply because, at present, there exists no observational evidence, i.e. they have never been detected. However, some valid solutions of the equations of General Relativity do point to their existence. Hence, wormholes are often called the “Metaphors for teaching General Relativity”. You may ask, “How are black holes and wormholes related?” Well, wormholes are simply two-ended black holes.

The term wormhole was coined by John Wheeler in 1957.

Introduction

The meaning of the word wormhole is very literal. It has its origin in worms digging tunnels in the ground to traverse from one place to another. In physics, a wormhole is a hypothetical feature of space-time that is, essentially, a “short-cut” through space-time. Whenever a massive object, such as a star, blows apart in a cataclysmic explosion called a supernova, it gives rise to a black hole. If this black hole does NOT have a singularity that narrows to a point but instead has considerable space around the singularity, it is called a wormhole. This black hole would fall through to another location, which may be anywhere in the universe or in an alternate universe.

                                 

What one needs to understand is that wormholes simply exist in theory. No black hole has been observed to contain a “wide” singularity. And obviously, we can hardly hope to ever venture even close to a black hole to observe it. Therefore wormholes are purely hypothetical till date. As mentioned earlier, certain solutions of equations of general relativity do contain wormholes. Here again, we know nothing about their stability. Nevertheless, wormholes continue to be supported by theoretical astrophysics and are an object of fascination for many scientists.

Understanding Black Holes

As I explained in my last blog, Black holes are regions in space having a gravitational field so strong that even light can’t escape from them and therefore they are invisible. Here are some characteristics:

Features of a Black Hole

No matter or information can flow from the inside of a black hole to the outside universe. The event horizon is the “surface” of a black hole. It is the perimeter at which the escape velocity is exactly the speed of light. The event horizon is marked by the light rays that are just unable to escape out of the black hole’s vicinity. At the centre of a black hole there is a singularity where gravity and escape velocity become infinite. The singularity has infinite density and space-time curvature. Astronomers suspect there are two classes of black holes: stellar mass and supermassive.

Stellar mass black holes have masses between 4 and 15 times the mass of the sun. Supermassive black holes have masses many billions of times greater than the Sun. Supermassive black holes are thought to form at the centres of active galaxies. We have one at the centre of our very own galaxy, the Milky Way.

Roger Penrose proposed the   Cosmic Censorship Hypothesis  which states that “God abhors a naked singularity.” This means that the singularities that occur in the universe are always “hidden” where we cannot see them, such as inside a black hole, hidden by its event horizon. It protects observers who are outside the black hole from the consequences of breakdown of predictability that occurs at the singularity. There are some solutions of the equations of General Relativity where it is possible for one to see a naked singularity and thus enable us to fall through the black hole and into a “wormhole”.

A wormhole is nothing but a black hole with “two ends”. It is a shortcut through space. What is unpredictable is where a wormhole might lead. In fact, there is no experimental evidence for the existence of wormholes. They have been proved to exist only theoretically. Wormholes have been the object of fancy of many sci-fi writers for the simple reason that they are the universe’s natural “time machines”. (Wormholes will be discussed in greater detail in a later blog!)

Detecting Black Holes

Black holes are invisible, but it is possible to detect them by studying their effects. Astronomers observing a star called Cygnus X-I saw that it was giving off enormous amounts of energy. They discovered that this huge, hot, blue star was being dragged around in a circle by an unseen object with a huge gravitational pull. That unseen object, astronomers believe, is a black hole, which is tearing gas from the star. The gas forms a whirling disc before plummeting into the black hole. As it falls, it travels faster and faster until it moves almost at the speed of light itself. Close to the black hole, the gas becomes so hot that it emits massive amounts of energy. Thus, the signatures of a black hole are detected.

Evidence

Only indirect evidence for black holes has been observed. Evidence for the existence of black holes comes from the observation of phenomena close to where a black hole might be.

Accretion disc

Matter falling toward a black hole, but still outside the event horizon, is thought to form a rapidly spinning accretion disc. Friction between different zones of the

accretion disc is possibly the cause of the emission of large amounts of X-rays. Narrow jets of particles moving at a velocity close to the speed of light are supposedly emitted along the polar axis of an accretion disc.

Other evidence

Stars that appear to be orbiting a region of space where no matter is visible may

be orbiting black holes. Gravitational lensing may distort light

from objects behind and far beyond a black hole.

Conclusion:

In his book, A Brief History of Time, Dr. Stephen Hawking goes on to explain why black holes aren’t exactly “sinister” objects found lurking in space. Black holes have several observable effects and of course, they form the centre of several galaxies including our own galaxy, the Milky Way. The most important possible “application” of a black hole could be long distance space travel by making use of wormholes. But, of course, this application is shrouded with complications and paradoxes. Until a time arrives, when the world of Star Trek truly comes alive, wormholes will only remain in our textbooks!