(A) Describe different methods to estimate the age of the Universe. (B) Explain the formation of Lunar Eclipse. (C) Explain the terms Dark Energy and Dark Matter. (D) Define the term Black Hole. What's expected inside it?

(A) Methods to estimate the age of the Universe

The Universe is estimated to be about 13.8 billion years old. Several independent methods support this.

Expansion rate: galaxies are moving apart, and the speed of recession is proportional to distance (Hubble's law). Running the expansion backward gives a rough age of about 1/H₀, where H₀ is the Hubble constant. A correction is then made for how the expansion rate has changed over time. Cosmic microwave background (CMB): this faint radiation left over from the early Universe has patterns that depend on its age and contents. Fitting these patterns to the standard cosmological model gave the Planck mission its figure of about 13.8 billion years. This is the most precise estimate. Oldest star clusters: the age of a globular cluster can be found from the stage its stars have reached in their life cycle. Since the Universe must be older than its oldest stars, this sets a minimum age of about 12 to 13 billion years. White dwarf cooling: white dwarfs cool at a predictable rate, so the faintest ones show how long they have existed. Radioactive dating of stars: the ratio of long-lived elements such as uranium and thorium in very old stars gives their approximate age.

(B) Formation of a lunar eclipse

A lunar eclipse occurs when the Earth comes directly between the Sun and the Moon, so that the Earth's shadow falls on the Moon. It occurs only at full moon. It does not happen every month because the Moon's orbit is tilted about 5° to the Earth's orbit, so the Moon usually passes above or below the shadow.

The Earth's shadow has a dark inner part, the umbra, and a lighter outer part, the penumbra. An eclipse is total when the whole Moon enters the umbra, partial when only part of it does, and penumbral when it passes through the penumbra alone. During totality, the Moon often turns coppery red, because the Earth's atmosphere scatters away blue light and bends red light into the shadow.

(C) Dark matter and dark energy

Dark matter is an unseen form of matter that does not emit, absorb or reflect light noticeably, and that is detected through its gravity. Evidence comes from galaxies rotating too fast for their visible mass, from gravitational lensing, and from patterns in the CMB. It makes up about 27% of the Universe's total mass-energy, and what it consists of remains unknown.

Dark energy is the name given to whatever is causing the expansion of the Universe to accelerate, as discovered in 1998 from observations of distant supernovae. It makes up about 68% of the total, and may be a property of space itself. Ordinary matter makes up only about 5%.

In short, dark matter pulls things together through gravity, while dark energy drives the expansion of space.

(D) Black hole

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. It generally forms when a massive star collapses at the end of its life. Supermassive black holes, millions to billions of times the Sun's mass, lie at the centres of most large galaxies. The first image of a black hole's shadow, in the galaxy M87, was released in 2019.

What is expected inside

Event horizon: this is the boundary of no return, rather than a solid surface. It cannot be seen through, so what lies within is known only from theory. Strong tidal forces: an object falling inward would be stretched, an effect called spaghettification. Singularity: general relativity predicts that matter collapses to a point of infinite density at the centre. Most physicists think this signals the limit of the theory, and that a quantum theory of gravity will be needed to describe what is actually there.