Researchers have pointed out at a "trend towards centralization". Although bitcoin can be sent directly from user to user, in practice intermediaries are widely used.[32]:220–222 Bitcoin miners join large mining pools to minimize the variance of their income.[32]:215, 219–222[115]:3[116] Because transactions on the network are confirmed by miners, decentralization of the network requires that no single miner or mining pool obtains 51% of the hashing power, which would allow them to double-spend coins, prevent certain transactions from being verified and prevent other miners from earning income.[117] As of 2013 just six mining pools controlled 75% of overall bitcoin hashing power.[117] In 2014 mining pool Ghash.io obtained 51% hashing power which raised significant controversies about the safety of the network. The pool has voluntarily capped their hashing power at 39.99% and requested other pools to act responsibly for the benefit of the whole network.[118] Between 2017 and 2019 over 70% of the hashing power and 90% of transactions were operating from China.[119]
Bitcoin, along with other cryptocurrencies, has been described as an economic bubble by at least eight Nobel Memorial Prize in Economic Sciences laureates, including Robert Shiller,[193] Joseph Stiglitz,[194] and Richard Thaler.[195][14] Noted Keyensian economist Paul Krugman wrote in his New York Times column criticizing bitcoin, calling it a bubble and a fraud;[196] and professor Nouriel Roubini of New York University called bitcoin the "mother of all bubbles."[197] Central bankers, including former Federal Reserve Chairman Alan Greenspan,[198] investors such as Warren Buffett,[199][200] and George Soros[201] have stated similar views, as have business executives such as Jamie Dimon and Jack Ma.[202]
In 1998, Wei Dai published a description of "b-money", characterized as an anonymous, distributed electronic cash system.[12] Shortly thereafter, Nick Szabo described bit gold.[13] Like bitcoin and other cryptocurrencies that would follow it, bit gold (not to be confused with the later gold-based exchange, BitGold) was described as an electronic currency system which required users to complete a proof of work function with solutions being cryptographically put together and published. A currency system based on a reusable proof of work was later created by Hal Finney who followed the work of Dai and Szabo.[citation needed]
The term altcoin has various similar definitions. Stephanie Yang of The Wall Street Journal defined altcoins as "alternative digital currencies,"[20] while Paul Vigna, also of The Wall Street Journal, described altcoins as alternative versions of bitcoin.[21] Aaron Hankins of the MarketWatch refers to any cryptocurrencies other than bitcoin as altcoins.[22]

Transaction fees for cryptocurrency depend mainly on the supply of network capacity at the time, versus the demand from the currency holder for a faster transaction. The currency holder can choose a specific transaction fee, while network entities process transactions in order of highest offered fee to lowest. Cryptocurrency exchanges can simplify the process for currency holders by offering priority alternatives and thereby determine which fee will likely cause the transaction to be processed in the requested time. 

Paul Krugman, Nobel Memorial Prize in Economic Sciences winner does not like bitcoin, has repeated numerous times that it is a bubble that will not last[92] and links it to Tulip mania.[93] American business magnate Warren Buffett thinks that cryptocurrency will come to a bad ending.[94] In October 2017, BlackRock CEO Laurence D. Fink called bitcoin an 'index of money laundering'.[95] "Bitcoin just shows you how much demand for money laundering there is in the world," he said.
Decentralized cryptocurrency is produced by the entire cryptocurrency system collectively, at a rate which is defined when the system is created and which is publicly known. In centralized banking and economic systems such as the Federal Reserve System, corporate boards or governments control the supply of currency by printing units of fiat money or demanding additions to digital banking ledgers. In case of decentralized cryptocurrency, companies or governments cannot produce new units, and have not so far provided backing for other firms, banks or corporate entities which hold asset value measured in it. The underlying technical system upon which decentralized cryptocurrencies are based was created by the group or individual known as Satoshi Nakamoto.[23]
On 25 March 2014, the United States Internal Revenue Service (IRS) ruled that bitcoin will be treated as property for tax purposes. This means bitcoin will be subject to capital gains tax.[64] In a paper published by researchers from Oxford and Warwick, it was shown that bitcoin has some characteristics more like the precious metals market than traditional currencies, hence in agreement with the IRS decision even if based on different reasons.[65]
Bitcoin is a digital currency, sometimes referred to as a cryptocurrency, best known as the world's first truly decentralized digital currency. Bitcoin is traded on a peer-to-peer basis with a distributed ledger called the Blockchain, and the Bitcoin exchange rate to the US Dollar and other major currencies is determined by supply and demand as with other global exchange rates. The traded value of Bitcoin has proven volatile through various booms and busts in demand. Ultimately, however, many see Bitcoin as a store of value against government-backed fiat currencies.
Two members of the Silk Road Task Force—a multi-agency federal task force that carried out the U.S. investigation of Silk Road—seized bitcoins for their own use in the course of the investigation.[69] DEA agent Carl Mark Force IV, who attempted to extort Silk Road founder Ross Ulbricht ("Dread Pirate Roberts"), pleaded guilty to money laundering, obstruction of justice, and extortion under color of official right, and was sentenced to 6.5 years in federal prison.[69] U.S. Secret Service agent Shaun Bridges pleaded guilty to crimes relating to his diversion of $800,000 worth of bitcoins to his personal account during the investigation, and also separately pleaded guilty to money laundering in connection with another cryptocurrency theft; he was sentenced to nearly eight years in federal prison.[70]
If the private key is lost, the bitcoin network will not recognize any other evidence of ownership;[32] the coins are then unusable, and effectively lost. For example, in 2013 one user claimed to have lost 7,500 bitcoins, worth $7.5 million at the time, when he accidentally discarded a hard drive containing his private key.[78] About 20% of all bitcoins are believed to be lost. They would have a market value of about $20 billion at July 2018 prices.[79] 

On 25 March 2014, the United States Internal Revenue Service (IRS) ruled that bitcoin will be treated as property for tax purposes. This means bitcoin will be subject to capital gains tax.[64] In a paper published by researchers from Oxford and Warwick, it was shown that bitcoin has some characteristics more like the precious metals market than traditional currencies, hence in agreement with the IRS decision even if based on different reasons.[65]
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