Astronomy

What is the largest moon in the solar system? It's Ganymede if counting only solid and liquid layers, otherwise it's Titan if including gas layers.

It turns out that the moon is moving away from Earth at 3.82 cm/year. That means that, eventually, it’ll result in Earth days lasting 25 hours in 211 million years. Atomic clocks show that Earth's day lengthens by about 17 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds. Although that translates to .0017 seconds over 100 years, that comes to a second every 58,800 years, or 211 million years for Earth to have 25-hour days.

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) (perigee) to 406,700 km (252,700 mi) (apogee), making the Moon's apparent size fluctuate.

The moon's mass is 1/81 of Earth's, being the 2nd densest among the planetary moons (after Io), and having the 2nd highest surface gravity, after Io.

2.46 billion years ago, the moon is calculated to be 37,280 miles (60,000 hm) closer (about 1.5 Earth circumferences), which would make our days roughly 17 hours long.

The history of discovering water on the moon.

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space.

1998 volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. India reached the Moon in 2008 for the 1st time with its Chandrayaan-1, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in Aug. 2018 for the 1st time "definitive evidence" for water-ice on the lunar surface.

In 2014 the 1st privately funded probe, the Manfred Memorial Moon Mission, reached the Moon.

Another Chinese rover mission, Chang'e 4, achieved the 1st landing on the Moon's far side in early 2019.

When will the sun blow up?

The sun, having a mass of 330,000 times that of Earth, converts 4 million tons of matter into energy every single second.

According to Live Science:

After the sun has burned through most of the hydrogen in its core, it will transition to its next phase as a red giant. At this point roughly 5 billion years in the future, the sun will stop generating heat via nuclear fusion, and its core will become unstable and contract, according to NASA. Meanwhile, the outer part of the sun, which will still contain hydrogen, will expand, glowing red as it cools. This expansion will gradually swallow the sun’s neighboring planets, Mercury and Venus, and ratchet up the sun’s solar winds to the point that they quash Earth’s magnetic field and strip off its atmosphere.

Of course, this will almost certainly be bad news for whatever life remains on our planet by that point — assuming any has survived the 10% increase in the sun’s brightness that is expected to vaporize Earth’s oceans in 1 billion to 1.5 billion years, according to a 2014 study published in Geophysical Research Letters. Within a few million years of this initial expansion, it’s likely that the sun will also consume the rocky remains of the Earth, according to a 2008 study published in the Monthly Notices of the Royal Astronomical Society.

History of finding life on other planets.

Sept. 2023 - breaking news in the history of astrobiology.

Chemicals that hint at life have been found in the atmosphere of an exoplanet 120 light years from Earth. The James Webb Telescope spotted signs of methane, carbon dioxide and dimethyl sulphide on “K2-18 b” – a planet nearly 9 times the size of Earth which sits in the Goldilocks zone of its star, where conditions are neither too hot, nor too cold for liquid water. NASA has said, with a radius 2.6 times that of the radius of Earth and is 8.6 times as massive as Earth, the K2-18 b’s large size means that the exoplanet’s interior has a large mantle of high-pressure ice.

The discovery of dimethyl sulfide is particularly intriguing because it is a molecule that is only produced by life on Earth, and is mostly emitted from phytoplankton in marine environments. Scientists believe that the planet may possess a hydrogen-rich atmosphere and a water ocean-covered surface that has the potential for life.

Dr. Nikku Madhusudhan, associate professor in Astrophysics at the University of Cambridge and lead author of a new paper on the discovery, said: “For the 1st time we have detected carbon-bearing molecules in the atmosphere of a habitable-zone planet. This has never happened before.”

K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and lies around 705 trillion miles from Earth in the constellation Leo.

Although the search for life has traditionally focused on rocky planets similar in composition to Earth, scientists are increasingly thinking that life could also exist on large watery worlds – dubbed Hycean exoplanets or sub-Neptunes.

“Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” said team member Subhajit Sarkar of Cardiff University.

The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be an ocean underneath a hydrogen-rich atmosphere on K2-18 b. NASA noted, however, that this ocean may be too hot to be habitable or to be liquid.

Although there is only a hint of dimethyl sulfide, future Webb observations are planned to confirm the findings.

K2-18 b was discovered in 2015, and due to its proximity to Earth and its dim star, it was considered a good candidate for detecting an atmosphere. The team of astrophysicists examined data from the Hubble Space Telescope collected in 2016 and 2017. While not ideal for detecting a wide range of molecular elements in distant exoplanets, Hubble is, however, capable of detecting water vapor.

The temperature of the planet is roughly –73 C to 46 C, which is similar to Earth. The range is so large because of various unknown factors, including the temperature of the star and the distance between the star and the planet and the planet's atmosphere and pressure, which is why it's unclear if water exists on the surface.

The 3 moons in our solar system most likely to have life.

Are Europa, Enceladus, and Titan.

A bit smaller that Earth’s moon, Europa’s ocean resides under an ice layer 10 to 15 miles (15 to 25 km) thick, with an estimated depth of 40 to 100 miles (60 to 150 km). “We know that Europa has a lot of the ingredients necessary for life, certainly for life as we know it. There’s water. There’s energy. There’s some amount of carbon material. But the habitability of Europa is one of the big questions that we want to understand,” said planetary scientist Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory, May 2018.

NASA’s Cassini spacecraft sampled plumes from Saturn’s ocean-bearing moon Enceladus that contained hydrogen from hydrothermal vents, an environment that may have given rise to life on Earth.

Titan is 5,149.46 kilometers (3,199.73 mi) in diameter, 1.06x that of Mercury, 1.48x that of the Moon, and 0.40 that of Earth.

Titan is the only known moon with a significant atmosphere (despite being the only moon in the solar system with a magnetic field), and its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth's. It was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the 1st direct images of Titan's surface were obtained.

In 1907 Spanish astronomer Josep Comas i Solà observed limb darkening of Titan, the 1st evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a spectroscopic technique to detect an atmosphere of methane.

Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), and hydrogen (0.1–0.2%), with trace amounts of other gases. There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane, and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium. The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog. Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan. On July 28, 2017, scientists reported that acrylonitrile, or vinyl cyanide, (C₂H₃CN), possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan.

History of finding exoplanets.

NASA's space observatory Keplar was launched in 2009, and completed its primary mission was completed in Nov. 2012.

By Sept. 2013, the number of known planets outside our solar system, called exoplanets, had grown to more than 900. That month, Japanese astronomers reported that the atmosphere of a large planet orbiting the star Gliese 1214, at 40 light-years from Earth, was probably rich in water.

-As of July 2015, confirmed more than 1,900 planets orbiting more than 1,100 stars, with more than 600 at least as massive as Jupiter.
-As of July 2017, confirmed over 3,500 planets orbiting ~2,200 stars, with 582 stars hosting more than 1 planet.
-As of July 2018, confirmed over 3,700 planets orbiting more than 2,816 stars, with 628 stars hosting more than 1 planet.
-As of Sept. 2019, confirmed over 4,107 planets orbiting more than 3,057 stars, with 667 stars hosting more than 1 planet.
-As of July 2021, confirmed over 4,777 planets orbiting more than 3,534 stars, with 785 stars hosting more than 1 planet.
-As of July 2022, confirmed over 5,069 planets orbiting more than 3,792 stars, with 829 stars hosting more than 1 planet.

The closest exoplanet, discovered Aug. 2016, Proxima b, orbits Proxima Centauri, the closest star, at 4.2 light-years away.

Solar flares.

solar flares are an intense burst of radiation coming from the release of magnetic energy associated with sunspots. NASA monitors flares primarily through X-rays and optical light. As of Jan. 2024, the last solar flare happened on Nov. 4, 2003, and was captured above the SOHO spacecraft.

Trivia.

Objects in low-Earth orbit, such as space shuttles, must maintain a speed of about 17,000 mph to remain in orbit. The exact speed depends on altitude. A typical shuttle orbits 115 to 400 miles above the Earth.

Pluto has 5 moons now. Pluto's moons Nix and Hydra, were discovered in 2005 and 2006. Kerberos and Styx were discovered in 2011 and 2012, and both officially named in 2013.

As of 2024, dark energy makes up 68.3% of the universe, dark matter 26.8%, and ordinary matter 4.9%. 85% of mass is dark matter.

Asteroids that crashed into Earth.

The ssteroid that slammed into Earth 66 million years ago and led to the extinction of dinosaurs, was estimated to be about 6.2 miles (10 km) in diameter and marked the last known large asteroid to hit our world. Planet killer asteroids are space rocks that are 1 kilometer across or larger and could have a devastating effect on life. 2024 YR4, is estimated to be 131 to 295 feet (40 to 90 meters). The amount of light reflected by the asteroid's surface is used to estimate how big it is.

In 1908, a 30 meter (98 feet) asteroid struck the Podkamennaya Tunguska River in a remote Siberian forest of Russia, according to the Planetary Society. The event leveled trees and destroyed forests across 830 square miles (2,150 square km).

And in 2013, a 20 meter (66 feet) asteroid entered Earth's atmosphere over Chelyabinsk, Russia. It exploded in the air, releasing 20 to 30x more energy than that of the 1st atomic bomb, generating brightness greater than the sun, exuding heat, damaging more than 7,000 buildings and injuring more than 1,000 people.

Gravitational waves from black holes.

The 1st detection of gravitational waves from black holes was made on Sept. 14, 2015, by the LIGO (Laser Interferometer Gravitational-Wave Observatory) detectors, but not announced publicly until Feb. 11, 2016, which marked a major breakthrough in astrophysics. The event, known as GW150914, involved the merger of 2 black holes about 1.3 billion light-years away.

Gravitational waves are ripples in spacetime caused by massive objects accelerating, and their detection confirmed a key prediction of Albert Einstein's theory of general relativity from 1915. The black holes in GW150914 were about 29x to 36x the mass of the sun, and their merger released energy equivalent to about 3 solar masses as gravitational waves.

This groundbreaking discovery opened a new way of observing the universe and has since been followed by many additional detections of gravitational waves from black hole and neutron star mergers. Upcoming detectors, like the Laser Interferometer Space Antenna (LISA), will unlock new opportunities by allowing us to detect mergers between stellar-mass black holes (tens of solar masses) and supermassive black holes (millions to billions of solar masses).

Cosmology, and early universe physics.

1st-Order Phase Transitions:

A 1st-order phase transition is characterized by a discontinuous change in some order parameter (such as the density or temperature of a system). In this type of transition, there is a latent heat release, and the new phase forms by bubble nucleation in the old phase.

Example: When water freezes into ice, it undergoes a 1st-order phase transition. As the temperature drops below freezing, tiny bubbles of ice form, grow, and eventually the entire body of water turns solid.

In the early universe, 1st-order phase transitions may have produced bubbles of vacuum, leading to phenomena like gravitational waves, which are ripples in spacetime.

2nd-Order Phase Transitions:

In a 2nd-order phase transition, the order parameter changes continuously, without the need for bubble nucleation. There is no latent heat involved, and the transition happens smoothly.

Example: A magnet loses its magnetism at a high enough temperature (the Curie point) in a continuous process. This is a 2nd-order phase transition where the system smoothly changes from 1 phase to another.

In cosmology, the electroweak phase transition could be 2nd-order, where the Higgs field smoothly transitions as the universe cools down.

In the context of the early universe, phase transitions are associated with changes in the vacuum state of fields, such as the Higgs field, during the universe's cooling. These transitions can release energy and generate new particles or forms of energy like dark matter, dark energy, or even dark radiation.

For example, the electroweak phase transition happened when the universe cooled down to a temperature where the electroweak force (which combines electromagnetic and weak nuclear forces) split into separate forces.

In cosmology, inflation refers to a period of extremely rapid expansion that occurred in the very early universe, right after the Big Bang, lasting for a tiny fraction of a second (from about 10^-36 seconds to 10^-32 seconds after the Big Bang).

At the end of inflation, the universe transitioned from a vacuum-dominated phase to a hot, dense state filled with particles, through a process called reheating. This is when the energy stored in the inflationary field (often called the inflaton field) was converted into particles, filling the universe with matter and radiation that would evolve into the universe we observe today.

During a 1st-order phase transition, as the universe cools, bubbles of the new phase (or vacuum) nucleate in the old phase. These bubbles expand and, depending on the dynamics, may or may not collide during inflation. If the bubbles don’t collide during inflation (which can happen if the phase transition occurs early), they can survive until after inflation. The vacuum energy of these bubbles can convert into radiation, potentially producing dark radiation.

The vacuum energy stored in the phase transition is released as the universe evolves, and in certain models, this energy could be channeled into light, weakly interacting particles that form dark radiation. This process naturally leads to isocurvature perturbations in the dark radiation component, since the phase transition generates spatially varying energy density fluctuations.

2nd-order phase transitions are continuous and smooth, meaning that the order parameter (like the Higgs field for the electroweak phase transition) changes gradually as the universe cools down. These phase transitions don't produce dramatic, local changes in the energy density of the universe. As a result, 2nd-order phase transitions do not generate gravitational waves because they lack the violent out-of-equilibrium processes necessary for such production.

1st-order phase transitions, on the other hand, involve a discontinuous change in the order parameter, where the universe transitions from 1 vacuum state to another by forming bubbles of the new vacuum phase. These bubbles nucleate and expand until they collide, creating a highly non-equilibrium, turbulent environment. This process can produce gravitational waves, as well as generate fluctuations in the energy density that can lead to isocurvature modes.

Isocurvature (non-adiabatic) fluctuations arise when different components of the universe’s energy density (like matter, radiation, or dark radiation) have different spatial distributions. In contrast, adiabatic fluctuations occur when all components have the same fractional energy density variation across space (meaning the ratios of different energy components remain constant).

A phase transition that generates dark radiation will typically produce isocurvature perturbations because the dark radiation density varies independently of the matter or photon densities, creating non-adiabatic fluctuations.

If a phase transition occurs during inflation, it can generate large bubbles of the new vacuum phase within the inflating universe. However, the inflationary expansion can stretch the bubbles apart so much that they don’t collide with each other during inflation, essentially freezing their spatial distribution until after inflation ends.

After inflation, as the universe reheats, the vacuum energy from the bubbles converts into radiation, which could include a dark sector contribution (such as dark radiation). The lack of bubble collisions during inflation ensures that the resulting dark radiation is not evenly distributed, thus generating isocurvature.

The isocurvature perturbations in dark radiation can lead to distinctive signatures in the cosmic microwave background (CMB) and the large-scale structure of the universe. If significant, these perturbations could alter the evolution of density fluctuations and affect the CMB power spectrum in ways that might be observable.

However, detecting dark radiation isocurvature is challenging because dark radiation is by definition weakly interacting, and its effects are subtle. Gravitational waves from 1st-order phase transitions might be an indirect way to probe the physics of the phase transitions that generate dark radiation.

Adiabatic vs. Isocurvature Perturbations

Adiabatic perturbations: In an adiabatic state, the energy density fluctuations across different components (e.g. photons, dark matter, dark energy) are uniform. That means if 1 component (like dark matter) increases, all others (like photons or baryons) increase in the same proportion. This is the standard perturbation pattern seeded by inflation.

Isocurvature perturbations: occur when the density fluctuations in 1 component (such as dark radiation) are independent of the others (like matter or photons). In this case, dark radiation could have density variations that differ from those of matter or photons, leading to independent spatial distributions.

The process of χ being converted into dark radiation.

Imagine that in the early universe, you have a field χ (kai), which is initially adiabatically coupled to the rest of the universe's energy content. This means that its fluctuations are aligned with the overall density perturbations (like photons and baryons). However, as the universe evolves, the χ field undergoes a phase transition or some other dynamic change, causing its energy density to be converted into dark radiation (such as a light, relativistic particle). Then, during a phase transition (1st-order or 2nd-order), the vacuum energy associated with the χ field gets released and converts into dark radiation. The transition generates spatially varying pockets or regions where χ contributes to dark radiation in a non-uniform way.

Isocurvature Generation:

Since the energy density of dark radiation now fluctuates independently from the photon or baryon densities, these fluctuations are no longer adiabatic. This introduces isocurvature perturbations in the dark radiation sector. The new dark radiation, resulting from the χ field’s evolution, exhibits density fluctuations that do not track the other energy components (like photons, dark matter, or baryons), creating a distinct signature.

Initially, the universe could have been dominated by adiabatic fluctuations where all components (including χ) followed the same density perturbation patterns. But once χ decays or transforms into dark radiation, its energy density is distributed differently, leading to non-adiabatic (isocurvature) perturbations.

These isocurvature fluctuations could arise from:

-The non-uniform decay of χ into dark radiation, which depends on where χ has higher or lower energy density.
-Bubble nucleation in a 1st-order phase transition, where the energy released from bubbles of different sizes and locations creates non-uniform distributions of dark radiation.

This reaction is irreversible.

In most theoretical frameworks of cosmology and particle physics, dark radiation cannot typically be converted back into the original field, such as χ (kai), after the phase transition or decay process has occurred. If χ decays into dark radiation through a process such as the production of axion-like particles or other light, weakly interacting particles, that decay releases energy, and the resulting particles quickly spread out through space. Once that energy is dispersed and the universe expands further, it becomes extremely difficult for that dispersed energy (in the form of dark radiation) to reassemble into the original χ field. Essentially, the entropy of the system increases, making the reverse process highly unlikely.

Dark radiation is composed of particles that typically don't interact strongly with the rest of the universe, including the χ field. It could be made of particles like neutrinos or axion-like particles, which, due to their weak interactions, would not easily be recaptured or recombined into a more structured form like a scalar field (χ). The energy density of dark radiation dilutes as the universe expands, making it even less likely for these particles to reform into anything resembling their original source.