The U.S. Supreme Court is back in session today after its summer break, and the new term is already shaping up to be an explosive one. Here are three cases to watch in the coming months:

1. Carpenter v. United States

The Fourth Amendment guarantees "the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures." Yet according to the U.S. Supreme Court's 1979 decision in Smith v. Maryland, "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."

Otherwise known as the third-party doctrine, this legal rule has been a great gift to law enforcement agencies on both the federal and state levels. Let's say the police want to know the email addresses of your correspondents, or the URLs of the websites you have visited. Under the third-party doctrine, the police do not need a search warrant (issued upon probable cause) to get that information from your internet service provider.

But doesn't the idea of granting vast warrantless search powers to the police run afoul of the bedrock protections enshrined in the Fourth Amendment?

The Supreme Court will grapple with those questions this term in Carpenter v. United States. At issue is whether the FBI violated the Fourth Amendment when it obtained, without a search warrant, the cellphone records of suspected armed robber Timothy Carpenter. With those records, law enforcement officials identified the cell towers that handled the suspect's calls and then proceeded to trace back his whereabouts during the time periods in which his alleged crimes were committed. That information was used against Carpenter in court.

According to Carpenter and his lawyers, "carrying a smartphone, checking for new emails from one's boss, updating the weather forecast, and downloading directions ought not license total surveillance of a person's entire life."

According to the Trump administration, "a cell-phone user has no reasonable expectation of privacy in business records created by his provider documenting the cell sites used to document his calls."

Oral arguments in Carpenter v. United States have not yet been scheduled.

2. Christie v. National Collegiate Athletic Association

According to the terms of the federal Professional and Amateur Sports Protection Act of 1992 (PASPA), it is illegal for "a governmental entity to sponsor, operate, advertise, promote, license, or authorize by law or compact" sports betting.

The state of New Jersey, however, went ahead and legalized sports betting in certain casinos and racetracks by partially lifting its existing ban on the practice. According to the National Collegiate Athletic Association, the National Basketball Association, the National Football League, the National Hockey League, the Office of the Commissioner of Baseball, and the Trump administration, the state's legalization effort is illegal under PASPA.

Put differently, Christie v. N.C.A.A. presents a clash between federalism and federal power.

"Never before has congressional power been construed to allow the federal government to dictate whether or to what extent a State may repeal, lift, or otherwise modulate its own state-law prohibitions on private conduct," New Jersey told the Supreme Court in its petition for certiorari. "And never before has federal law been enforced to command a State to give effect to a state law that the State has chosen to repeal."

PASPA is "an unremarkable exercise of Congress' settled power to regulate commerce in sports gambling," the sports leagues counter in their brief in opposition to the state's petition. "PASPA is a straightforward exercise of Congress' power to preempt the operation of state laws that conflict with federal policy on matters within Congress' purview."

The Trump administration favors federal power in this matter too. PASPA "does not violate the Tenth Amendment because it neither compels States to regulate according to federal standards nor requires state officials to administer federal law," the administration told the Court in an amicus brief.

Oral arguments in Christie v. N.C.A.A. have not yet been scheduled.

3. Janus v. American Federation of State, County, and Municipal Employees, Council 31

In 1977's Abood v. Detroit Board of Education, the Supreme Court said it was permissible for state and local governments to require public-sector workers to pay union fees as a condition of government employment even if those workers are not union members. The Court reasoned that such mandatory fees passed muster because they prevented non-members from "free riding" on the union's collective bargaining efforts.

At issue in Janus v. American Federation of State, County, and Municipal Employees, Council 31, is whether Abood should be overturned.

The case originates with Mark Janus, an Illinois state employee who objects to paying mandatory fees to a union that he refuses to join. Janus argues that by making him contribute to the union's coffers, the state is violating his First Amendment rights by forcing him to support political speech and activism that he does not wish to support.

"The Court should take this case," Janus and his lawyers argue in their petition for certiorari, "to overrule Abood and declare [mandatory public-sector union] fees unconstitutional."

The American Federation of State, County, and Municipal Employees takes the opposite view. "Abood's rule is sound and underlies important and longstanding tenets of this Court's First Amendment jurisprudence," the union told the Court in its brief in opposition to that petition. "At its core, Abood acknowledged that certain labor-relations interests justify the small intrusion on employees' First Amendment interests that fair-share payments represent."

If the basic contours of this dispute sound familiar, that's because the Supreme Court tackled a nearly identical matter last term in the case of Friedrichs v. California Teachers Association. But after the death of Justice Antonin Scalia, the Court stalemated in that case and tied 4-4. It is quite likely that if Scalia had not died, Abood would have been overturned by a 5-4 decision.

That means the decisive vote in Janus is almost certainly in the hands of Scalia's replacement. All eyes will be on Justice Neil Gorsuch when this case comes around.

Oral arguments in Janus v. AFSCME, Council 31, have not yet been scheduled.