Early and modern theories

The Big Bang model describes the early universe as extremely hot and dense; it then expanded and cooled. This is more accurate than describing its initial condition only by size.

As the gas-and-dust cloud collapsed, it formed a protostar. The protostar became the Sun after conditions in its core allowed sustained nuclear fusion.

The Tidal Hypothesis proposed that the gravitational pull of a passing star drew material from the Sun, from which planets could form.

In the Planetesimal Hypothesis, small solid bodies called planetesimals accumulated and joined to form larger bodies, including planets.

The correct answer is A. On large scales, the expansion of space increases the average separation between galaxies that are not held together by gravity. Bound systems, such as some nearby galaxy pairs, do not necessarily expand with the universe.

The correct answer is A. A cloud of gas and dust collapsed and flattened into a rotating disk. Dust and rock in the disk gathered into small bodies, which accreted to form planets.

The correct answer is A. The Nebular and Tidal hypotheses are historical explanations proposed for the origin of the Solar System and Earth; the other pairs concern unrelated topics.

The correct answer is A. The Big Bang model describes the universe evolving from an early hot, dense state and expanding over time. The other options concern processes on Earth, not the universe as a whole.

The correct answer is A. The Big Bang model describes the universe expanding from an early hot, dense state. A passing star is associated with a historical tidal hypothesis, while planetesimals and a solar nebula concern Solar System formation.

The correct answer is A. In the historical Tidal Hypothesis, a star passing close to the Sun was thought to raise tides and pull material from it, from which planets could form. This is a historical proposal, not the modern accepted model of Solar System formation.

The correct answer is A. The Planetesimal Hypothesis proposes that small solid bodies, called planetesimals, accumulated to form planets. The other choices describe cosmic expansion or an Earth process, not this hypothesis.

The correct answer is A. The Nebular Hypothesis describes the Solar System forming from a rotating cloud of gas and dust, which collapsed and flattened into a disk. The other choices are unrelated to this astronomical explanation.

The correct answer is C. The cosmic microwave background is relic radiation from the hot early universe, now observed as microwaves. Its existence is an important line of evidence supporting the Big Bang model.

The correct answer is A. In the modern solar-nebula model, a rotating cloud of gas and dust flattened into a disk. The Sun formed near its centre, and material in the surrounding disk accumulated into planets. The other statements do not describe this model.

Many early theories were developed when observations and scientific evidence were limited. As a result, they could not explain all the features of the Earth and solar system that later evidence helped scientists investigate.

Observations and measurements allow scientists to test whether a theory fits what is seen and measured. Evidence can therefore support a theory, lead to its revision, or show that it needs to be replaced.

In the standard cosmological account, the universe began in the Big Bang and then expanded and cooled. Much later, matter gathered under gravity to form stars and galaxies; this option gives that broad sequence in simplified form.

Laplace is associated with the nebular hypothesis, not the Big Bang theory. The other pairings are standard associations: Chamberlin and Moulton with the planetesimal hypothesis, Kant with an early nebular hypothesis, and Hubble with evidence for the expansion of the universe.

The tidal hypothesis, associated with Jeans and Jeffreys, proposed that a nearby star's gravitational pull drew material from the Sun. The material was then thought to condense into planets.

A theory's central idea and its associated scientist or scientists help distinguish it from other theories. For example, linking Kant with an early nebular hypothesis helps identify the theory being described.

Many early proposals were made with fewer observations available. Modern theories can be tested against a much wider range of observations and measurements, although scientific theories remain open to revision when new evidence appears.

The correct answer is A, accretion. Small particles collide and stick together, gradually building larger bodies that can grow into planets.

The correct answer is B, the Big Bang Theory. It describes the universe expanding from an initially very hot, dense state; the question does not require the idea of a literal point.

The correct answer is C, the Sun. In the solar nebula model, matter concentrated and condensed at the centre to form the Sun, while planets formed from material around it.

Nebular Hypothesis links the solar system with a nebula of gas and dust. Exam tip: rotating gas dust cloud points to nebular hypothesis.